CODE_E% = P% LOAD_E% = LOAD% + P% - CODE%ELITE E FILE Produces the binary file ELTE.bin that gets loaded by elite-bcfs.asm.EQUB '(' EOR 164 EQUB 'C' EOR 164 EQUB ')' EOR 164 EQUB 'B' EOR 164 EQUB 'e' EOR 164 EQUB 'l' EOR 164 EQUB 'l' EOR 164 EQUB '/' EOR 164 EQUB 'B' EOR 164 EQUB 'r' EOR 164 EQUB 'a' EOR 164 EQUB 'b' EOR 164 EQUB 'e' EOR 164 EQUB 'n' EOR 164 EQUB '1' EOR 164 EQUB '9' EOR 164 EQUB '8' EOR 164 EQUB '4' EOR 164Name: Authors' names [View individually] Type: Variable Category: Copy protection Summary: The authors' names and a copyright notice, buried in the code

This copyright notice is not used anywhere and it is obfuscated by EOR'ing each character with 164, but presumably the authors wanted their names buried in the code somewhere. Though they do also have recursive token 94, which reads "BY D.BRABEN & I.BELL" and can be displayed on the title screen using the "X" configuration option, so this isn't the only author name easter egg in the game. It contains the following text: (C)Bell/Braben1984.cpl LDX #5 \ First we need to backup the seeds in QQ15, so set up \ a counter in X to cover three 16-bit seeds (i.e. \ 6 bytes) .TT53 LDA QQ15,X \ Copy byte X from QQ15 to QQ19 STA QQ19,X DEX \ Decrement the loop counter BPL TT53 \ Loop back for the next byte to backup LDY #3 \ Step 1: Now that the seeds are backed up, we can \ start the name-generation process. We will either \ need to loop three or four times, so for now set \ up a counter in Y to loop four times BIT QQ15 \ Check bit 6 of w0_lo, which is stored in QQ15 BVS P%+3 \ If bit 6 is set then skip over the next instruction DEY \ Bit 6 is clear, so we only want to loop three times, \ so decrement the loop counter in Y STY T \ Store the loop counter in T .TT55 LDA QQ15+5 \ Step 2: Load w2_hi, which is stored in QQ15+5, and AND #%00011111 \ extract bits 0-4 by AND'ing with %11111 BEQ P%+7 \ If all those bits are zero, then skip the following \ 2 instructions to go to step 3 ORA #%10000000 \ We now have a number in the range 1-31, which we can \ easily convert into a two-letter token, but first we \ need to add 128 (or set bit 7) to get a range of \ 129-159 JSR TT27 \ Print the two-letter token in A JSR TT54 \ Step 3: twist the seeds in QQ15 DEC T \ Decrement the loop counter BPL TT55 \ Loop back for the next two letters LDX #5 \ We have printed the system name, so we can now \ restore the seeds we backed up earlier. Set up a \ counter in X to cover three 16-bit seeds (i.e. 6 \ bytes) .TT56 LDA QQ19,X \ Copy byte X from QQ19 to QQ15 STA QQ15,X DEX \ Decrement the loop counter BPL TT56 \ Loop back for the next byte to restore RTS \ Once all the seeds are restored, return from the \ subroutineName: cpl [View individually] Type: Subroutine Category: Text Summary: Print the selected system name Deep dive: Generating system names

Print control code 3 (the selected system name, i.e. the one in the crosshairs in the Short-range Chart). Other entry points: cmn-1 Contains an RTS.cmn LDY #0 \ Set up a counter in Y, starting from 0 .QUL4 LDA NA%,Y \ The commander's name is stored at NA%, so load the \ Y-th character from NA% CMP #13 \ If we have reached the end of the name, return from BEQ ypl-1 \ the subroutine (ypl-1 points to the RTS below) JSR TT26 \ Print the character we just loaded INY \ Increment the loop counter BNE QUL4 \ Loop back for the next character RTS \ Return from the subroutineName: cmn [View individually] Type: Subroutine Category: Text Summary: Print the commander's name

Print control code 4 (the commander's name). Other entry points: ypl-1 Contains an RTS.ypl LDA MJ \ Check the mis-jump flag at MJ, and if it is non-zero BNE cmn-1 \ then we are in witchspace, and witchspace doesn't have \ a system name, so return from the subroutine (cmn-1 \ contains an RTS) JSR TT62 \ Call TT62 below to swap the three 16-bit seeds in \ QQ2 and QQ15 (before the swap, QQ2 contains the seeds \ for the current system, while QQ15 contains the seeds \ for the selected system) JSR cpl \ Call cpl to print out the system name for the seeds \ in QQ15 (which now contains the seeds for the current \ system) \ Now we fall through into the TT62 subroutine, which \ will swap QQ2 and QQ15 once again, so everything goes \ back into the right place, and the RTS at the end of \ TT62 will return from the subroutine .TT62 LDX #5 \ Set up a counter in X for the three 16-bit seeds we \ want to swap (i.e. 6 bytes) .TT78 LDA QQ15,X \ Swap byte X between QQ2 and QQ15 LDY QQ2,X STA QQ2,X STY QQ15,X DEX \ Decrement the loop counter BPL TT78 \ Loop back for the next byte to swap RTS \ Once all bytes are swapped, return from the \ subroutineName: ypl [View individually] Type: Subroutine Category: Text Summary: Print the current system name

Print control code 2 (the current system name)..tal CLC \ We don't want to print the galaxy number with a \ decimal point, so clear the C flag for pr2 to take as \ an argument LDX GCNT \ Load the current galaxy number from GCNT into X INX \ Add 1 to the galaxy number, as the galaxy numbers \ are 0-7 internally, but we want to display them as \ galaxy 1 through 8 JMP pr2 \ Jump to pr2, which prints the number in X to a width \ of 3 figures, left-padding with spaces to a width of \ 3, and once done, return from the subroutine (as pr2 \ ends with an RTS)Name: tal [View individually] Type: Subroutine Category: Text Summary: Print the current galaxy numbe

Print control code 1 (the current galaxy number, right-aligned to width 3)..fwl LDA #105 \ Print recursive token 105 ("FUEL") followed by a JSR TT68 \ colon LDX QQ14 \ Load the current fuel level from QQ14 SEC \ We want to print the fuel level with a decimal point, \ so set the C flag for pr2 to take as an argument JSR pr2 \ Call pr2, which prints the number in X to a width of \ 3 figures (i.e. in the format x.x, which will always \ be exactly 3 characters as the maximum fuel is 7.0) LDA #195 \ Print recursive token 35 ("LIGHT YEARS") followed by JSR plf \ a newline .PCASH \ This label is not used but is in the original source LDA #119 \ Print recursive token 119 ("CASH:" then control code BNE TT27 \ 0, which prints cash levels, then " CR" and newline)Name: fwl [View individually] Type: Subroutine Category: Text Summary: Print fuel and cash levels

Print control code 5 ("FUEL: ", fuel level, " LIGHT YEARS", newline, "CASH:", control code 0)..csh LDX #3 \ We are going to use the BPRNT routine to print out \ the current amount of cash, which is stored as a \ 32-bit number at location CASH. BPRNT prints out \ the 32-bit number stored in K, so before we call \ BPRNT, we need to copy the four bytes from CASH into \ K, so first we set up a counter in X for the 4 bytes .pc1 LDA CASH,X \ Copy byte X from CASH to K STA K,X DEX \ Decrement the loop counter BPL pc1 \ Loop back for the next byte to copy LDA #9 \ We want to print the cash using up to 9 digits STA U \ (including the decimal point), so store this in U \ for BRPNT to take as an argument SEC \ We want to print the fuel level with a decimal point, \ so set the C flag for BRPNT to take as an argument JSR BPRNT \ Print the amount of cash to 9 digits with a decimal \ point LDA #226 \ Print recursive token 66 (" CR") followed by a \ newline by falling through into plfName: csh [View individually] Type: Subroutine Category: Text Summary: Print the current amount of cash

Print control code 0 (the current amount of cash, right-aligned to width 9, followed by " CR" and a newline)..plf JSR TT27 \ Print the text token in A JMP TT67 \ Jump to TT67 to print a newline and return from the \ subroutine using a tail callName: plf [View individually] Type: Subroutine Category: Text Summary: Print a text token followed by a newline

Arguments: A The text token to be printed.TT68 JSR TT27 \ Print the text token in A and fall through into TT73 \ to print a colonName: TT68 [View individually] Type: Subroutine Category: Text Summary: Print a text token followed by a colon

Arguments: A The text token to be printed.TT73 LDA #':' \ Set A to ASCII ":" and fall through into TT27 to \ actually print the colon.TT27 TAX \ Copy the token number from A to X. We can then keep \ decrementing X and testing it against zero, while \ keeping the original token number intact in A; this \ effectively implements a switch statement on the \ value of the token BEQ csh \ If token = 0, this is control code 0 (current amount \ of cash and newline), so jump to csh BMI TT43 \ If token > 127, this is either a two-letter token \ (128-159) or a recursive token (160-255), so jump \ to TT43 to process tokens DEX \ If token = 1, this is control code 1 (current BEQ tal \ galaxy number), so jump to tal DEX \ If token = 2, this is control code 2 (current system BEQ ypl \ name), so jump to ypl DEX \ If token > 3, skip the following instruction BNE P%+5 JMP cpl \ This token is control code 3 (selected system name) \ so jump to cpl DEX \ If token = 4, this is control code 4 (commander BEQ cmn \ name), so jump to cmm DEX \ If token = 5, this is control code 5 (fuel, newline, BEQ fwl \ cash, newline), so jump to fwl DEX \ If token > 6, skip the following 3 instructions BNE P%+7 LDA #%10000000 \ This token is control code 6 (switch to Sentence STA QQ17 \ Case), so set bit 7 of QQ17 to switch to Sentence Case RTS \ and return from the subroutine as we are done DEX \ If token > 8, skip the following 2 instructions DEX BNE P%+5 STX QQ17 \ This token is control code 8 (switch to ALL CAPS), so RTS \ set QQ17 to 0 to switch to ALL CAPS and return from \ the subroutine as we are done DEX \ If token = 9, this is control code 9 (tab to column BEQ crlf \ 21 and print a colon), so jump to crlf CMP #96 \ By this point, token is either 7, or in 10-127. BCS ex \ Check token number in A and if token >= 96, then the \ token is in 96-127, which is a recursive token, so \ jump to ex, which prints recursive tokens in this \ range (i.e. where the recursive token number is \ correct and doesn't need correcting) CMP #14 \ If token < 14, skip the following 2 instructions BCC P%+6 CMP #32 \ If token < 32, then this means token is in 14-31, so BCC qw \ this is a recursive token that needs 114 adding to it \ to get the recursive token number, so jump to qw \ which will do this \ By this point, token is either 7 (beep) or in 10-13 \ (line feeds and carriage returns), or in 32-95 \ (ASCII letters, numbers and punctuation) LDX QQ17 \ Fetch QQ17, which controls letter case, into X BEQ TT74 \ If QQ17 = 0, then ALL CAPS is set, so jump to TT27 \ to print this character as is (i.e. as a capital) BMI TT41 \ If QQ17 has bit 7 set, then we are using Sentence \ Case, so jump to TT41, which will print the \ character in upper or lower case, depending on \ whether this is the first letter in a word BIT QQ17 \ If we get here, QQ17 is not 0 and bit 7 is clear, so BVS TT46 \ either it is bit 6 that is set, or some other flag in \ QQ17 is set (bits 0-5). So check whether bit 6 is set. \ If it is, then ALL CAPS has been set (as bit 7 is \ clear) but bit 6 is still indicating that the next \ character should be printed in lower case, so we need \ to fix this. We do this with a jump to TT46, which \ will print this character in upper case and clear bit \ 6, so the flags are consistent with ALL CAPS going \ forward \ If we get here, some other flag is set in QQ17 (one \ of bits 0-5 is set), which shouldn't happen in this \ version of Elite. If this were the case, then we \ would fall through into TT42 to print in lower case, \ which is how printing all words in lower case could \ be supported (by setting QQ17 to 1, say)Name: TT27 [View individually] Type: Subroutine Category: Text Summary: Print a text token

Print a text token (i.e. a character, control code, two-letter token or recursive token). See variable QQ18 for a discussion of the token system used in Elite. Arguments: A The text token to be printed.TT42 CMP #'A' \ If A < ASCII "A", then this is punctuation, so jump BCC TT44 \ to TT26 (via TT44) to print the character as is, as \ we don't care about the character's case CMP #'Z'+1 \ If A >= (ASCII "Z" + 1), then this is also BCS TT44 \ punctuation, so jump to TT26 (via TT44) to print the \ character as is, as we don't care about the \ character's case ADC #32 \ Add 32 to the character, to convert it from upper to \ to lower case .TT44 JMP TT26 \ Print the character in AName: TT42 [View individually] Type: Subroutine Category: Text Summary: Print a letter in lower case

Arguments: A The character to be printed. Can be one of the following: * 7 (beep) * 10-13 (line feeds and carriage returns) * 32-95 (ASCII capital letters, numbers and punctuation) Other entry points: TT44 Jumps to TT26 to print the character in A (used to enable us to use a branch instruction to jump to TT26).TT41 \ If we get here, then QQ17 has bit 7 set, so we are in \ Sentence Case BIT QQ17 \ If QQ17 also has bit 6 set, jump to TT45 to print BVS TT45 \ this character in lower case \ If we get here, then QQ17 has bit 6 clear and bit 7 \ set, so we are in Sentence Case and we need to print \ the next letter in upper case CMP #'A' \ If A < ASCII "A", then this is punctuation, so jump BCC TT74 \ to TT26 (via TT44) to print the character as is, as \ we don't care about the character's case PHA \ Otherwise this is a letter, so store the token number TXA \ Set bit 6 in QQ17 (X contains the current QQ17) ORA #%1000000 \ so the next letter after this one is printed in lower STA QQ17 \ case PLA \ Restore the token number into A BNE TT44 \ Jump to TT26 (via TT44) to print the character in A \ (this BNE is effectively a JMP as A will never be \ zero)Name: TT41 [View individually] Type: Subroutine Category: Text Summary: Print a letter according to Sentence Case

The rules for printing in Sentence Case are as follows: * If QQ17 bit 6 is set, print lower case (via TT45) * If QQ17 bit 6 clear, then: * If character is punctuation, just print it * If character is a letter, set QQ17 bit 6 and print letter as a capital Arguments: A The character to be printed. Can be one of the following: * 7 (beep) * 10-13 (line feeds and carriage returns) * 32-95 (ASCII capital letters, numbers and punctuation) X Contains the current value of QQ17 QQ17 Bit 7 is set.qw ADC #114 \ This is a recursive token in the range 0-95, so add BNE ex \ 114 to the argument to get the token number 128-145 \ and jump to ex to print itName: qw [View individually] Type: Subroutine Category: Text Summary: Print a recursive token in the range 128-145

Print a recursive token where the token number is in 128-145 (so the value passed to TT27 is in the range 14-31). Arguments: A A value from 128-145, which refers to a recursive token in the range 14-31.crlf LDA #21 \ Set the X-column in XC to 21 STA XC BNE TT73 \ Jump to TT73, which prints a colon (this BNE is \ effectively a JMP as A will never be zero)Name: crlf [View individually] Type: Subroutine Category: Text Summary: Tab to column 21 and print a colon

Print control code 9 (tab to column 21 and print a colon). The subroutine name is pretty misleading, as it doesn't have anything to do with carriage returns or line feeds..TT45 \ If we get here, then QQ17 has bit 6 and 7 set, so we \ are in Sentence Case and we need to print the next \ letter in lower case CPX #255 \ If QQ17 = 255 then printing is disabled, so return BEQ TT48 \ from the subroutine (as TT48 contains an RTS) CMP #'A' \ If A >= ASCII "A", then jump to TT42, which will BCS TT42 \ print the letter in lowercase \ Otherwise this is not a letter, it's punctuation, so \ this is effectively a word break. We therefore fall \ through to TT46 to print the character and set QQ17 \ to ensure the next word starts with a capital letterName: TT45 [View individually] Type: Subroutine Category: Text Summary: Print a letter in lower case

This routine prints a letter in lower case. Specifically: * If QQ17 = 255, abort printing this character as printing is disabled * If this is a letter then print in lower case * Otherwise this is punctuation, so clear bit 6 in QQ17 and print Arguments: A The character to be printed. Can be one of the following: * 7 (beep) * 10-13 (line feeds and carriage returns) * 32-95 (ASCII capital letters, numbers and punctuation) X Contains the current value of QQ17 QQ17 Bits 6 and 7 are set.TT46 PHA \ Store the token number TXA \ Clear bit 6 in QQ17 (X contains the current QQ17) so AND #%10111111 \ the next letter after this one is printed in upper STA QQ17 \ case PLA \ Restore the token number into A \ Now fall through into TT74 to print the characterName: TT46 [View individually] Type: Subroutine Category: Text Summary: Print a character and switch to capitals

Print a character and clear bit 6 in QQ17, so that the next letter that gets printed after this will start with a capital letter. Arguments: A The character to be printed. Can be one of the following: * 7 (beep) * 10-13 (line feeds and carriage returns) * 32-95 (ASCII capital letters, numbers and punctuation) X Contains the current value of QQ17 QQ17 Bits 6 and 7 are set.TT74 JMP TT26 \ Print the character in AName: TT74 [View individually] Type: Subroutine Category: Text Summary: Print a character

Arguments: A The character to be printed.TT43 CMP #160 \ If token >= 160, then this is a recursive token, so BCS TT47 \ jump to TT47 below to process it AND #127 \ This is a two-letter token with number 128-159. The ASL A \ set of two-letter tokens is stored in a lookup table \ at QQ16, with each token taking up two bytes, so to \ convert this into the token's position in the table, \ we subtract 128 (or just clear bit 7) and multiply \ by 2 (or shift left) TAY \ Transfer the token's position into Y so we can look \ up the token using absolute indexed mode LDA QQ16,Y \ Get the first letter of the token and print it JSR TT27 LDA QQ16+1,Y \ Get the second letter of the token CMP #'?' \ If the second letter of the token is a question mark BEQ TT48 \ then this is a one-letter token, so just return from \ the subroutine without printing (as TT48 contains an \ RTS) JMP TT27 \ Print the second letter and return from the \ subroutine .TT47 SBC #160 \ This is a recursive token in the range 160-255, so \ subtract 160 from the argument to get the token \ number 0-95 and fall through into ex to print itName: TT43 [View individually] Type: Subroutine Category: Text Summary: Print a two-letter token or recursive token 0-95

Print a two-letter token, or a recursive token where the token number is in 0-95 (so the value passed to TT27 is in the range 160-255). Arguments: A One of the following: * 128-159 (two-letter token) * 160-255 (the argument to TT27 that refers to a recursive token in the range 0-95).ex TAX \ Copy the token number into X LDA #LO(QQ18) \ Set V, V+1 to point to the recursive token table at STA V \ location QQ18 LDA #HI(QQ18) STA V+1 LDY #0 \ Set a counter Y to point to the character offset \ as we scan through the table TXA \ Copy the token number back into A, so both A and X \ now contain the token number we want to print BEQ TT50 \ If the token number we want is 0, then we have \ already found the token we are looking for, so jump \ to TT50, otherwise start working our way through the \ null-terminated token table until we find the X-th \ token .TT51 LDA (V),Y \ Fetch the Y-th character from the token table page \ we are currently scanning BEQ TT49 \ If the character is null, we've reached the end of \ this token, so jump to TT49 INY \ Increment character pointer and loop back round for BNE TT51 \ the next character in this token, assuming Y hasn't \ yet wrapped around to 0 INC V+1 \ If it has wrapped round to 0, we have just crossed BNE TT51 \ into a new page, so increment V+1 so that V points \ to the start of the new page .TT49 INY \ Increment the character pointer BNE TT59 \ If Y hasn't just wrapped around to 0, skip the next \ instruction INC V+1 \ We have just crossed into a new page, so increment \ V+1 so that V points to the start of the new page .TT59 DEX \ We have just reached a new token, so decrement the \ token number we are looking for BNE TT51 \ Assuming we haven't yet reached the token number in \ X, look back up to keep fetching characters .TT50 \ We have now reached the correct token in the token \ table, with Y pointing to the start of the token as \ an offset within the page pointed to by V, so let's \ print the recursive token. Because recursive tokens \ can contain other recursive tokens, we need to store \ our current state on the stack, so we can retrieve \ it after printing each character in this token TYA \ Store the offset in Y on the stack PHA LDA V+1 \ Store the high byte of V (the page containing the PHA \ token we have found) on the stack, so the stack now \ contains the address of the start of this token LDA (V),Y \ Load the character at offset Y in the token table, \ which is the next character of this token that we \ want to print EOR #35 \ Tokens are stored in memory having been EOR'd with 35 \ (see variable QQ18 for details), so we repeat the \ EOR to get the actual character to print JSR TT27 \ Print the text token in A, which could be a letter, \ number, control code, two-letter token or another \ recursive token PLA \ Restore the high byte of V (the page containing the STA V+1 \ token we have found) into V+1 PLA \ Restore the offset into Y TAY INY \ Increment Y to point to the next character in the \ token we are printing BNE P%+4 \ If Y is zero then we have just crossed into a new INC V+1 \ page, so increment V+1 so that V points to the start \ of the new page LDA (V),Y \ Load the next character we want to print into A BNE TT50 \ If this is not the null character at the end of the \ token, jump back up to TT50 to print the next \ character, otherwise we are done printing .TT48 RTS \ Return from the subroutineName: ex [View individually] Type: Subroutine Category: Text Summary: Print a recursive token

This routine works its way through the recursive tokens that are stored in tokenised form in memory at &0400 to &06FF, and when it finds token number A, it prints it. Tokens are null-terminated in memory and fill three pages, but there is no lookup table as that would consume too much memory, so the only way to find the correct token is to start at the beginning and look through the table byte by byte, counting tokens as we go until we are in the right place. This approach might not be terribly speed efficient, but it is certainly memory-efficient. For details of the tokenisation system, see variable QQ18. Arguments: A The recursive token to be printed, in the range 0-148 Other entry points: TT48 Contains an RTS.EX2 LDA INWK+31 \ Set bits 5 and 7 of the ship's byte #31 to denote that ORA #%10100000 \ the ship is exploding and has been killed STA INWK+31 RTS \ Return from the subroutine .DOEXP LDA INWK+31 \ If bit 6 of the ship's byte #31 is clear, then the AND #%01000000 \ ship is not already exploding so there is no existing BEQ P%+5 \ explosion cloud to remove, so skip the following \ instruction JSR PTCLS \ Call PTCLS to remove the existing cloud by drawing it \ again LDA INWK+6 \ Set T = z_lo STA T LDA INWK+7 \ Set A = z_hi, so (A T) = z CMP #32 \ If z_hi < 32, skip the next two instructions BCC P%+6 LDA #&FE \ Set A = 254 and jump to yy (this BNE is effectively a BNE yy \ JMP, as A is never zero) ASL T \ Shift (A T) left twice ROL A ASL T ROL A SEC \ And then shift A left once more, inserting a 1 into ROL A \ bit 0 \ Overall, the above multiplies A by 8 and makes sure it \ is at least 1, to leave a one-byte distance in A. We \ can use this as the distance for our cloud, to ensure \ that the explosion cloud is visible even for ships \ that blow up a long way away .yy STA Q \ Store the distance to the explosion in Q LDY #1 \ Fetch byte #1 of the ship line heap, which contains LDA (XX19),Y \ the cloud counter ADC #4 \ Add 4 to the cloud counter, so it ticks onwards every \ we redraw it BCS EX2 \ If the addition overflowed, jump up to EX2 to update \ the explosion flags and return from the subroutine STA (XX19),Y \ Store the updated cloud counter in byte #1 of the ship \ line heap JSR DVID4 \ Calculate the following: \ \ (P R) = 256 * A / Q \ = 256 * cloud counter / distance \ \ We are going to use this as our cloud size, so the \ further away the cloud, the smaller it is, and as the \ cloud counter ticks onward, the cloud expands LDA P \ Set A = P, so we now have: \ \ (A R) = 256 * cloud counter / distance CMP #&1C \ If A < 28, skip the next two instructions BCC P%+6 LDA #&FE \ Set A = 254 and skip the following (this BNE is BNE LABEL_1 \ effectively a JMP as A is never zero) ASL R \ Shift (A R) left three times to multiply by 8 ROL A ASL R ROL A ASL R ROL A \ Overall, the above multiplies (A R) by 8 to leave a \ one-byte cloud size in A, given by the following: \ \ A = 8 * cloud counter / distance .LABEL_1 DEY \ Decrement Y to 0 STA (XX19),Y \ Store the cloud size in byte #0 of the ship line heap LDA INWK+31 \ Clear bit 6 of the ship's byte #31 to denote that the AND #%10111111 \ explosion has not yet been drawn STA INWK+31 AND #%00001000 \ If bit 3 of the ship's byte #31 is clear, then nothing BEQ TT48 \ is being drawn on-screen for this ship anyway, so \ return from the subroutine (as TT48 contains an RTS) LDY #2 \ Otherwise it's time to draw an explosion cloud, so LDA (XX19),Y \ fetch byte #2 of the ship line heap into Y, which we TAY \ set to the explosion count for this ship (i.e. the \ number of vertices used as origins for explosion \ clouds) \ \ The explosion count is stored as 4 * n + 6, where n is \ the number of vertices, so the following loop copies \ the coordinates of the first n vertices from the heap \ at XX3, which is where we stored all the visible \ vertex coordinates in part 8 of the LL9 routine, and \ sticks them in the ship line heap pointed to by XX19, \ starting at byte #7 (so it leaves the first 6 bytes of \ the ship line heap alone) .EXL1 LDA XX3-7,Y \ Copy byte Y-7 from the XX3 heap, into the Y-th byte of STA (XX19),Y \ the ship line heap DEY \ Decrement the loop counter CPY #6 \ Keep copying vertex coordinates into the ship line BNE EXL1 \ heap until Y = 6 (which will copy n vertices, where n \ is the number of vertices we should be exploding) LDA INWK+31 \ Set bit 6 of the ship's byte #31 to denote that the ORA #%01000000 \ explosion has been drawn (as it's about to be) STA INWK+31 .PTCLS \ This part of the routine actually draws the explosion \ cloud LDY #0 \ Fetch byte #0 of the ship line heap, which contains LDA (XX19),Y \ the cloud size we stored above, and store it in Q STA Q INY \ Increment the index in Y to point to byte #1 LDA (XX19),Y \ Fetch byte #1 of the ship line heap, which contains \ the cloud counter. We are now going to process this \ into the number of particles in each vertex's cloud BPL P%+4 \ If the cloud counter < 128, then we are in the first \ half of the cloud's existence, so skip the next \ instruction EOR #&FF \ Flip the value of A so that in the second half of the \ cloud's existence, A counts down instead of up LSR A \ Divide A by 8 so that is has a maximum value of 15 LSR A LSR A ORA #1 \ Make sure A is at least 1 and store it in U, to STA U \ give us the number of particles in the explosion for \ each vertex INY \ Increment the index in Y to point to byte #2 LDA (XX19),Y \ Fetch byte #2 of the ship line heap, which contains STA TGT \ the explosion count for this ship (i.e. the number of \ vertices used as origins for explosion clouds) and \ store it in TGT LDA RAND+1 \ Fetch the current random number seed in RAND+1 and PHA \ store it on the stack, so we can re-randomise the \ seeds when we are done LDY #6 \ Set Y = 6 to point to the byte before the first vertex \ coordinate we stored on the ship line heap above (we \ increment it below so it points to the first vertex) .EXL5 LDX #3 \ We are about to fetch a pair of coordinates from the \ ship line heap, so set a counter in X for 4 bytes .EXL3 INY \ Increment the index in Y so it points to the next byte \ from the coordinate we are copying LDA (XX19),Y \ Copy the Y-th byte from the ship line heap to the X-th STA K3,X \ byte of K3 DEX \ Decrement the X index BPL EXL3 \ Loop back to EXL3 until we have copied all four bytes \ The above loop copies the vertex coordinates from the \ ship line heap to K3, reversing them as we go, so it \ sets the following: \ \ K3+3 = x_lo \ K3+2 = x_hi \ K3+1 = y_lo \ K3+0 = y_hi STY CNT \ Set CNT to the index that points to the next vertex on \ the ship line heap LDY #2 \ Set Y = 2, which we will use to point to bytes #3 to \ #6, after incrementing it \ This next loop copies bytes #3 to #6 from the ship \ line heap into the four random number seeds in RAND to \ RAND+3, EOR'ing them with the vertex index so they are \ different for every vertex. This enables us to \ generate random numbers for drawing each vertex that \ are random but repeatable, which we need when we \ redraw the cloud to remove it \ \ Note that we haven't actually set the values of bytes \ #3 to #6 in the ship line heap, so we have no idea \ what they are, we just use what's already there. But \ the fact that those bytes are stored for this ship \ means we can repeat the random generation of the \ cloud, which is the important bit .EXL2 INY \ Increment the index in Y so it points to the next \ random number seed to copy LDA (XX19),Y \ Fetch the Y-th byte from the ship line heap EOR CNT \ EOR with the vertex index, so the seeds are different \ for each vertex STA &FFFD,Y \ Y is going from 3 to 6, so this stores the four bytes \ in memory locations &00, &01, &02 and &03, which are \ the memory locations of RAND through RAND+3 CPY #6 \ Loop back to EXL2 until Y = 6, which means we have BNE EXL2 \ copied four bytes LDY U \ Set Y to the number of particles in the explosion for \ each vertex, which we stored in U above. We will now \ use this as a loop counter to iterate through all the \ particles in the explosion .EXL4 JSR DORND2 \ Set ZZ to a random number (also restricts the STA ZZ \ value of RAND+2 so that bit 0 is always 0) LDA K3+1 \ Set (A R) = (y_hi y_lo) STA R \ = y LDA K3 JSR EXS1 \ Set (A X) = (A R) +/- random * cloud size \ = y +/- random * cloud size BNE EX11 \ If A is non-zero, the particle is off-screen as the \ coordinate is bigger than 255), so jump to EX11 to do \ the next particle CPX #2*Y-1 \ If X > the y-coordinate of the bottom of the screen, BCS EX11 \ the particle is off the bottom of the screen, so jump \ to EX11 to do the next particle \ Otherwise X contains a random y-coordinate within the \ cloud STX Y1 \ Set Y1 = our random y-coordinate within the cloud LDA K3+3 \ Set (A R) = (x_hi x_lo) STA R LDA K3+2 JSR EXS1 \ Set (A X) = (A R) +/- random * cloud size \ = x +/- random * cloud size BNE EX4 \ If A is non-zero, the particle is off-screen as the \ coordinate is bigger than 255), so jump to EX11 to do \ the next particle \ Otherwise X contains a random x-coordinate within the \ cloud LDA Y1 \ Set A = our random y-coordinate within the cloud JSR PIXEL \ Draw a point at screen coordinate (X, A) with the \ point size determined by the distance in ZZ .EX4 DEY \ Decrement the loop counter for the next particle BPL EXL4 \ Loop back to EXL4 until we have done all the particles \ in the cloud LDY CNT \ Set Y to the index that points to the next vertex on \ the ship line heap CPY TGT \ If Y < TGT, which we set to the explosion count for BCC EXL5 \ this ship (i.e. the number of vertices used as origins \ for explosion clouds), loop back to EXL5 to do a cloud \ for the next vertex PLA \ Restore the current random number seed to RAND+1 that STA RAND+1 \ we stored at the start of the routine LDA K%+6 \ Store the z_lo coordinate for the planet (which will STA RAND+3 \ be pretty random) in the RAND+3 seed RTS \ Return from the subroutine .EX11 JSR DORND2 \ Set A and X to random numbers (also restricts the \ value of RAND+2 so that bit 0 is always 0) JMP EX4 \ We just skipped a particle, so jump up to EX4 to do \ the next one .EXS1 \ This routine calculates the following: \ \ (A X) = (A R) +/- random * cloud size \ \ returning with the flags set for the high byte in A STA S \ Store A in S so we can use it later JSR DORND2 \ Set A and X to random numbers (also restricts the \ value of RAND+2 so that bit 0 is always 0) ROL A \ Set A = A * 2 BCS EX5 \ If bit 7 of A was set (50% chance), jump to EX5 JSR FMLTU \ Set A = A * Q / 256 \ = random << 1 * projected cloud size / 256 ADC R \ Set (A X) = (S R) + A TAX \ = (S R) + random * projected cloud size \ \ where S contains the argument A, starting with the low \ bytes LDA S \ And then the high bytes ADC #0 RTS \ Return from the subroutine .EX5 JSR FMLTU \ Set T = A * Q / 256 STA T \ = random << 1 * projected cloud size / 256 LDA R \ Set (A X) = (S R) - T SBC T \ TAX \ where S contains the argument A, starting with the low \ bytes LDA S \ And then the high bytes SBC #0 RTS \ Return from the subroutineName: DOEXP [View individually] Type: Subroutine Category: Drawing ships Summary: Draw an exploding ship Deep dive: Drawing explosion clouds.SOS1 JSR msblob \ Update the dashboard's missile indicators to all be \ green/cyan LDA #127 \ Set the pitch and roll counters to 127 (no damping STA INWK+29 \ so the planet's rotation doesn't slow down) STA INWK+30 LDA tek \ Set A = 128 or 130 depending on bit 1 of the system's AND #%00000010 \ tech level in tek ORA #%10000000 JMP NWSHP \ Add a new planet to our local bubble of universe, \ with the planet type defined by A (128 is a planet \ with an equator and meridian, 130 is a planet with \ a crater)Name: SOS1 [View individually] Type: Subroutine Category: Universe Summary: Update the missile indicators, set up the planet data block

Update the missile indicators, and set up a data block for the planet, but only setting the pitch and roll counters to 127 (no damping)..SOLAR LSR FIST \ Halve our legal status in FIST, making us less bad, \ and moving bit 0 into the C flag (so every time we \ arrive in a new system, our legal status improves a \ bit) JSR ZINF \ Call ZINF to reset the INWK ship workspace, which \ doesn't affect the C flag LDA QQ15+1 \ Fetch w0_hi AND #%00000111 \ Extract bits 0-2 (which also happen to determine the \ economy), which will be between 0 and 7 ADC #6 \ Add 6 + C, and divide by 2, to get a result between 3 LSR A \ and 7 STA INWK+8 \ Store the result in z_sign in byte #6 ROR A \ Halve A, rotating in the C flag, which was previously STA INWK+2 \ bit 0 of w0_hi + 6 + C, so when this is stored in both STA INWK+5 \ x_sign and y_sign, it moves the planet to the upper \ right or lower left JSR SOS1 \ Call SOS1 to set up the planet's data block and add it \ to FRIN, where it will get put in the first slot as \ it's the first one to be added to our local bubble of \ this new system's universe LDA QQ15+3 \ Fetch w1_hi, extract bits 0-2, set bits 0 and 7 and AND #%00000111 \ store in z_sign, so the sun is behind us at a distance ORA #%10000001 \ of 1 to 7 STA INWK+8 LDA QQ15+5 \ Fetch w2_hi, extract bits 0-1 and store in x_sign and AND #%00000011 \ y_sign, so the sun is either dead in our rear laser STA INWK+2 \ crosshairs, or off to the top left by a distance of 1 STA INWK+1 \ or 2 when we look out the back LDA #0 \ Set the pitch and roll counters to 0 (no rotation) STA INWK+29 STA INWK+30 LDA #129 \ Set A = 129, the "ship" type for the sun JSR NWSHP \ Call NWSHP to set up the sun's data block and add it \ to FRIN, where it will get put in the second slot as \ it's the second one to be added to our local bubble \ of this new system's universeName: SOLAR [View individually] Type: Subroutine Category: Universe Summary: Set up various aspects of arriving in a new system

Halve our legal status, update the missile indicators, and set up data blocks and slots for the planet and sun..NWSTARS LDA QQ11 \ If this is not a space view, jump to WPSHPS to skip \ORA MJ \ the initialisation of the SX, SY and SZ tables. The OR BNE WPSHPS \ instruction is commented out in the original source, \ but it would have the effect of also skipping the \ initialisation if we had mis-jumped into witchspaceName: NWSTARS [View individually] Type: Subroutine Category: Stardust Summary: Initialise the stardust field

This routine is called when the space view is initialised in routine LOOK1..nWq LDY NOSTM \ Set Y to the current number of stardust particles, so \ we can use it as a counter through all the stardust .SAL4 JSR DORND \ Set A and X to random numbers ORA #8 \ Set A so that it's at least 8 STA SZ,Y \ Store A in the Y-th particle's z_hi coordinate at \ SZ+Y, so the particle appears in front of us STA ZZ \ Set ZZ to the particle's z_hi coordinate JSR DORND \ Set A and X to random numbers STA SX,Y \ Store A in the Y-th particle's x_hi coordinate at \ SX+Y, so the particle appears in front of us STA X1 \ Set X1 to the particle's x_hi coordinate JSR DORND \ Set A and X to random numbers STA SY,Y \ Store A in the Y-th particle's y_hi coordinate at \ SY+Y, so the particle appears in front of us STA Y1 \ Set Y1 to the particle's y_hi coordinate JSR PIXEL2 \ Draw a stardust particle at (X1,Y1) with distance ZZ DEY \ Decrement the counter to point to the next particle of \ stardust BNE SAL4 \ Loop back to SAL4 until we have randomised all the \ stardust particles \ Fall through into WPSHPS to clear the scanner and \ reset the LSO blockName: nWq [View individually] Type: Subroutine Category: Stardust Summary: Create a random cloud of stardust

Create a random cloud of stardust containing the correct number of dust particles, i.e. NOSTM of them, which is 3 in witchspace and 18 (#NOST) in normal space. Also clears the scanner and initialises the LSO block. This is called by the DEATH routine when it displays our untimely demise..WPSHPS LDX #0 \ Set up a counter in X to work our way through all the \ ship slots in FRIN .WSL1 LDA FRIN,X \ Fetch the ship type in slot X BEQ WS2 \ If the slot contains 0 then it is empty and we have \ checked all the slots (as they are always shuffled \ down in the main loop to close up and gaps), so jump \ to WS2 as we are done BMI WS1 \ If the slot contains a ship type with bit 7 set, then \ it contains the planet or the sun, so jump down to WS1 \ to skip this slot, as the planet and sun don't appear \ on the scanner STA TYPE \ Store the ship type in TYPE JSR GINF \ Call GINF to get the address of the data block for \ ship slot X and store it in INF LDY #31 \ We now want to copy the first 32 bytes from the ship's \ data block into INWK, so set a counter in Y .WSL2 LDA (INF),Y \ Copy the Y-th byte from the data block pointed to by STA INWK,Y \ INF into the Y-th byte of INWK workspace DEY \ Decrement the counter to point at the next byte BPL WSL2 \ Loop back to WSL2 until we have copied all 32 bytes STX XSAV \ Store the ship slot number in XSAV while we call SCAN JSR SCAN \ Call SCAN to plot this ship on the scanner, which will \ remove it as it's plotted with EOR logic LDX XSAV \ Restore the ship slot number from XSAV into X LDY #31 \ Clear bits 3, 4 and 6 in the ship's byte #31, which LDA (INF),Y \ stops drawing the ship on-screen (bit 3), hides it AND #%10100111 \ from the scanner (bit 4) and stops any lasers firing STA (INF),Y \ at it (bit 6) .WS1 INX \ Increment X to point to the next ship slot BNE WSL1 \ Loop back up to process the next slot (this BNE is \ effectively a JMP as X will never be zero) .WS2 LDX #&FF \ Set LSX2 = LSY2 = &FF to clear the ball line heap STX LSX2 STX LSY2 \ Fall through into FLFLLS to reset the LSO blockName: WPSHPS [View individually] Type: Subroutine Category: Dashboard Summary: Clear the scanner, reset the ball line and sun line heaps

Remove all ships from the scanner, reset the sun line heap at LSO, and reset the ball line heap at LSX2 and LSY2..FLFLLS LDY #2*Y-1 \ #Y is the y-coordinate of the centre of the mode 4 \ space view, so this sets Y as a counter for the number \ of lines in the space view (i.e. 191), which is also \ the number of lines in the LSO block LDA #0 \ Set A to 0 so we can zero-fill the LSO block .SAL6 STA LSO,Y \ Set the Y-th byte of the LSO block to 0 DEY \ Decrement the counter BNE SAL6 \ Loop back until we have filled all the way to LSO+1 DEY \ Decrement Y to value of &FF (as we exit the above loop \ with Y = 0) STY LSX \ Set the first byte of the LSO block, which has its own \ label LSX, to &FF, to indicate that the sun line heap \ is empty RTS \ Return from the subroutineName: FLFLLS [View individually] Type: Subroutine Category: Drawing suns Summary: Reset the sun line heap

Reset the sun line heap at LSO by zero-filling it and setting the first byte to &FF. Returns: A A is set to 0.DET1 LDA #6 \ Set A to 6 so we can update 6845 register R6 below SEI \ Disable interrupts so we can update the 6845 STA SHEILA+&00 \ Set 6845 register R6 to the value in X. Register R6 STX SHEILA+&01 \ is the "vertical displayed" register, which sets the \ number of rows shown on the screen CLI \ Re-enable interrupts RTS \ Return from the subroutineName: DET1 [View individually] Type: Subroutine Category: Screen mode Summary: Hide the dashboard (for when we die)

Set the screen to show the number of text rows given in X. This is used when we are killed, as reducing the number of rows from the usual 31 to 24 has the effect of hiding the dashboard, leaving a monochrome image of ship debris and explosion clouds. Increasing the rows back up to 31 makes the dashboard reappear, as the dashboard's screen memory doesn't get touched by this process. Arguments: X The number of text rows to display on the screen (24 will hide the dashboard, 31 will make it reappear) Returns A A is set to 6DEX \ Increment the shield value so that it doesn't go past \ a maximum of 255 RTS \ Return from the subroutine .SHD INX \ Increment the shield value BEQ SHD-2 \ If the shield value is 0 then this means it was 255 \ before, which is the maximum value, so jump to SHD-2 \ to bring it back down to 258 and return \ Otherwise fall through into DENGY to drain our energy \ to pay for all this shield chargingName: SHD [View individually] Type: Subroutine Category: Flight Summary: Charge a shield and drain some energy from the energy banks

Charge up a shield, and if it needs charging, drain some energy from the energy banks. Arguments: X The value of the shield to recharge.DENGY DEC ENERGY \ Decrement the energy banks in ENERGY PHP \ Save the flags on the stack BNE P%+5 \ If the energy levels are not yet zero, skip the \ following instruction INC ENERGY \ The minimum allowed energy level is 1, and we just \ reached 0, so increment ENERGY back to 1 PLP \ Restore the flags from the stack, so we return with \ the Z flag from the DEC instruction above RTS \ Return from the subroutineName: DENGY [View individually] Type: Subroutine Category: Flight Summary: Drain some energy from the energy banks

Returns: Z flag Set if we have no energy left, clear otherwise.COMPAS JSR DOT \ Call DOT to redraw (i.e. remove) the current compass \ dot LDA SSPR \ If we are inside the space station safe zone, jump to BNE SP1 \ SP1 to draw the space station on the compass JSR SPS1 \ Otherwise we need to draw the planet on the compass, \ so first call SPS1 to calculate the vector to the \ planet and store it in XX15 JMP SP2 \ Jump to SP2 to draw XX15 on the compass, returning \ from the subroutine with a tail call.SPS2 ASL A \ Set X = |A| * 2, and set the C flag to the sign bit of TAX \ A LDA #0 \ Set Y to have the sign bit from A in bit 7, with the ROR A \ rest of its bits zeroed, so Y now contains the sign of TAY \ the original argument LDA #20 \ Set Q = 20 STA Q TXA \ Copy X into A, so A now contains the argument A * 2 JSR DVID4 \ Calculate the following: \ \ P = A / Q \ = |argument A| * 2 / 20 \ = |argument A| / 10 LDX P \ Set X to the result TYA \ If the sign of the original argument A is negative, BMI LL163 \ jump to LL163 to flip the sign of the result LDY #0 \ Set the high byte of the result to 0, as the result is \ positive RTS \ Return from the subroutine .LL163 LDY #&FF \ The result is negative, so set the high byte to &FF TXA \ Flip the low byte and add 1 to get the negated low EOR #&FF \ byte, using two's complement TAX INX RTS \ Return from the subroutineName: SPS2 [View individually] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (Y X) = A / 10

Calculate the following, where A is a signed 8-bit integer and the result is a signed 16-bit integer: (Y X) = A / 10 Returns: C flag The C flag is cleared.SPS4 LDX #8 \ First we need to copy the space station's coordinates \ into K3, so set a counter to copy the first 9 bytes \ (the 3-byte x, y and z coordinates) from the station's \ data block at K% + NI% into K3 .SPL1 LDA K%+NI%,X \ Copy the X-th byte from the station's data block at STA K3,X \ K% + NI% to the X-th byte of K3 DEX \ Decrement the loop counter BPL SPL1 \ Loop back to SPL1 until we have copied all 9 bytes JMP TAS2 \ Call TAS2 to build XX15 from K3, returning from the \ subroutine using a tail callName: SPS4 [View individually] Type: Subroutine Category: Maths (Geometry) Summary: Calculate the vector to the space station

Calculate the vector between our ship and the space station and store it in XX15..SP1 JSR SPS4 \ Call SPS4 to calculate the vector to the space station \ and store it in XX15 \ Fall through into SP2 to draw XX15 on the compassName: SP1 [View individually] Type: Subroutine Category: Dashboard Summary: Draw the space station on the compass.SP2 LDA XX15 \ Set A to the x-coordinate of the planet or station to \ show on the compass, which will be in the range -96 to \ +96 as the vector has been normalised JSR SPS2 \ Set (Y X) = A / 10, so X will be from -9 to +9, which \ is the x-offset from the centre of the compass of the \ dot we want to draw. Returns with the C flag clear TXA \ Set COMX = 195 + X, as 186 is the pixel x-coordinate ADC #195 \ of the leftmost dot possible on the compass, and X can STA COMX \ be -9, which would be 195 - 9 = 186. This also means \ that the highest value for COMX is 195 + 9 = 204, \ which is the pixel x-coordinate of the rightmost dot \ in the compass... but the compass dot is actually two \ pixels wide, so the compass dot can overlap the right \ edge of the compass, but not the left edge LDA XX15+1 \ Set A to the y-coordinate of the planet or station to \ show on the compass, which will be in the range -96 to \ +96 as the vector has been normalised JSR SPS2 \ Set (Y X) = A / 10, so X will be from -9 to +9, which \ is the y-offset from the centre of the compass of the \ dot we want to draw. Returns with the C flag clear STX T \ Set COMY = 204 - X, as 203 is the pixel y-coordinate LDA #204 \ of the centre of the compass, the C flag is clear, SBC T \ and the y-axis needs to be flipped around (because STA COMY \ when the planet or station is above us, and the \ vector is therefore positive, we want to show the dot \ higher up on the compass, which has a smaller pixel \ y-coordinate). So this calculation does this: \ \ COMY = 204 - X - (1 - 0) = 203 - X LDA #&F0 \ Set A to a 4-pixel mode 5 byte row in colour 2 \ (yellow/white), the colour for when the planet or \ station in the compass is in front of us LDX XX15+2 \ If the z-coordinate of the XX15 vector is positive, BPL P%+4 \ skip the following instruction LDA #&FF \ The z-coordinate of XX15 is negative, so the planet or \ station is behind us and the compass dot should be in \ green/cyan, so set A to a 4-pixel mode 5 byte row in \ colour 3 STA COMC \ Store the compass colour in COMC \ Fall through into DOT to draw the dot on the compassName: SP2 [View individually] Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass, given the planet/station vector

Draw a dot on the compass to represent the planet or station, whose normalised vector is in XX15. XX15 to XX15+2 The normalised vector to the planet or space station, stored as x in XX15, y in XX15+1 and z in XX15+2.DOT LDA COMY \ Set Y1 = COMY, the y-coordinate of the dot STA Y1 LDA COMX \ Set X1 = COMX, the x-coordinate of the dot STA X1 LDA COMC \ Set COL = COMC, the mode 5 colour byte for the dot STA COL CMP #&F0 \ If COL is &F0 then the dot is in front of us and we BNE CPIX2 \ want to draw a double-height dot, so if it isn't &F0 \ jump to CPIX2 to draw a single-height dot \ Otherwise fall through into CPIX4 to draw a double- \ height dotName: DOT [View individually] Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass

Draw a dot on the compass. Arguments: COMX The screen pixel x-coordinate of the dot COMY The screen pixel y-coordinate of the dot COMC The colour and thickness of the dot: * &F0 = a double-height dot in yellow/white, for when the object in the compass is in front of us * &FF = a single-height dot in green/cyan, for when the object in the compass is behind us.CPIX4 JSR CPIX2 \ Call CPIX2 to draw a single-height dash at (X1, Y1) DEC Y1 \ Decrement Y1 \ Fall through into CPIX2 to draw a second single-height \ dash on the pixel row above the first one, to create a \ double-height dotName: CPIX4 [View individually] Type: Subroutine Category: Drawing pixels Summary: Draw a double-height dot on the dashboard

Draw a double-height mode 5 dot (2 pixels high, 2 pixels wide). Arguments: X1 The screen pixel x-coordinate of the bottom-left corner of the dot Y1 The screen pixel y-coordinate of the bottom-left corner of the dot COL The colour of the dot as a mode 5 character row byte.CPIX2 LDA Y1 \ Fetch the y-coordinate into A \.CPIX \ This label is commented out in the original source. It \ would provide a new entry point with A specifying the \ y-coordinate instead of Y1, but it isn't used anywhere TAY \ Store the y-coordinate in Y LSR A \ Set A = A / 8, so A now contains the character row we LSR A \ need to draw in (as each character row contains 8 LSR A \ pixel rows) ORA #&60 \ Each character row in Elite's screen mode takes up one \ page in memory (256 bytes), so we now OR with &60 to \ get the page containing the dash (see the comments in \ routine TT26 for more discussion about calculating \ screen memory addresses STA SCH \ Store the screen page in the high byte of SC(1 0) LDA X1 \ Each character block covers 8 screen x-coordinates, so AND #%11111000 \ to get the address of the first byte in the character \ block that we need to draw into, as an offset from the \ start of the row we clear bits 0-2 STA SC \ Store the address of the character block in the low \ byte of SC(1 0), so now SC(1 0) points to the \ character block we need to draw into TYA \ Set Y to just bits 0-2 of the y-coordinate, which will AND #%00000111 \ be the number of the pixel row we need to draw into TAY \ within the character block LDA X1 \ Copy bits 0-1 of X to bits 1-2 of X1, and clear the C AND #%00000110 \ flag in the process (using the LSR). X will now be LSR A \ a value between 0 and 3, and will be the pixel number TAX \ in the character row for the left pixel in the dash. \ This is because each character row is one byte that \ contains 4 pixels, but covers 8 screen coordinates, so \ this effectively does the division by 2 that we need LDA CTWOS,X \ Fetch a mode 5 1-pixel byte with the pixel position AND COL \ at X, and AND with the colour byte so that pixel takes \ on the colour we want to draw (i.e. A is acting as a \ mask on the colour byte) EOR (SC),Y \ Draw the pixel on-screen using EOR logic, so we can STA (SC),Y \ remove it later without ruining the background that's \ already on-screen LDA CTWOS+1,X \ Fetch a mode 5 1-pixel byte with the pixel position \ at X+1, so we can draw the right pixel of the dash BPL CP1 \ The CTWOS table has an extra row at the end of it that \ repeats the first value, %10001000, so if we have not \ fetched that value, then the right pixel of the dash \ is in the same character block as the left pixel, so \ jump to CP1 to draw it LDA SC \ Otherwise the left pixel we drew was at the last ADC #8 \ position of four in this character block, so we add STA SC \ 8 to the screen address to move onto the next block \ along (as there are 8 bytes in a character block). \ The C flag was cleared above, so this ADC is correct LDA CTWOS+1,X \ Refetch the mode 5 1-pixel byte, as we just overwrote \ A (the byte will still be the fifth byte from the \ table, which is correct as we want to draw the \ leftmost pixel in the next character along as the \ dash's right pixel) .CP1 AND COL \ Draw the dash's right pixel according to the mask in EOR (SC),Y \ A, with the colour in COL, using EOR logic, just as STA (SC),Y \ above RTS \ Return from the subroutineName: CPIX2 [View individually] Type: Subroutine Category: Drawing pixels Summary: Draw a single-height dot on the dashboard Deep dive: Drawing colour pixels in mode 5

Draw a single-height mode 5 dash (1 pixel high, 2 pixels wide). Arguments: X1 The screen pixel x-coordinate of the dash Y1 The screen pixel y-coordinate of the dash COL The colour of the dash as a mode 5 character row byte.OOPS STA T \ Store the amount of damage in T LDY #8 \ Fetch byte #8 (z_sign) for the ship attacking us, and LDX #0 \ set X = 0 LDA (INF),Y BMI OO1 \ If A is negative, then we got hit in the rear, so jump \ to OO1 to process damage to the aft shield LDA FSH \ Otherwise the forward shield was damaged, so fetch the SBC T \ shield strength from FSH and subtract the damage in T BCC OO2 \ If the C flag is clear then this amount of damage was \ too much for the shields, so jump to OO2 to set the \ shield level to 0 and start taking damage directly \ from the energy banks STA FSH \ Store the new value of the forward shield in FSH RTS \ Return from the subroutine .OO2 \LDX #0 \ This instruction is commented out in the original \ source, and isn't required as X is set to 0 above STX FSH \ Set the forward shield to 0 BCC OO3 \ Jump to OO3 to start taking damage directly from the \ energy banks (this BCC is effectively a JMP as the C \ flag is clear, as we jumped to OO2 with a BCC) .OO1 LDA ASH \ The aft shield was damaged, so fetch the shield SBC T \ strength from ASH and subtract the damage in T BCC OO5 \ If the C flag is clear then this amount of damage was \ too much for the shields, so jump to OO5 to set the \ shield level to 0 and start taking damage directly \ from the energy banks STA ASH \ Store the new value of the aft shield in ASH RTS \ Return from the subroutine .OO5 \LDX #0 \ This instruction is commented out in the original \ source, and isn't required as X is set to 0 above STX ASH \ Set the aft shield to 0 .OO3 ADC ENERGY \ A is negative and contains the amount by which the STA ENERGY \ damage overwhelmed the shields, so this drains the \ energy banks by that amount (and because the energy \ banks are shown over four indicators rather than one, \ but with the same value range of 0-255, energy will \ appear to drain away four times faster than the \ shields did) BEQ P%+4 \ If we have just run out of energy, skip the next \ instruction to jump straight to our death BCS P%+5 \ If the C flag is set, then subtracting the damage from \ the energy banks didn't underflow, so we had enough \ energy to survive, and we can skip the next \ instruction to make a sound and take some damage JMP DEATH \ Otherwise our energy levels are either 0 or negative, \ and in either case that means we jump to our DEATH, \ returning from the subroutine using a tail call JSR EXNO3 \ We didn't die, so call EXNO3 to make the sound of a \ collision JMP OUCH \ And jump to OUCH to take damage and return from the \ subroutine using a tail callName: OOPS [View individually] Type: Subroutine Category: Flight Summary: Take some damage

We just took some damage, so reduce the shields if we have any, or reduce the energy levels and potentially take some damage to the cargo if we don't. Arguments: A The amount of damage to take INF The address of the ship block for the ship that attacked us, or the ship that we just ran into.SPS3 LDA K%+1,X \ Copy x_hi into K3+X STA K3,X LDA K%+2,X \ Set A = Y = x_sign TAY AND #%01111111 \ Set K3+1 = |x_sign| STA K3+1,X TYA \ Set K3+2 = the sign of x_sign AND #%10000000 STA K3+2,X RTS \ Return from the subroutineName: SPS3 [View individually] Type: Subroutine Category: Maths (Geometry) Summary: Copy a space coordinate from the K% block into K3

Copy one of the planet's coordinates into the corresponding location in the temporary variable K3. The high byte and absolute value of the sign byte are copied into the first two K3 bytes, and the sign of the sign byte is copied into the highest K3 byte. The comments below are written for the x-coordinate. Arguments: X Determines which coordinate to copy, and to where: * X = 0 copies (x_sign, x_hi) into K3(2 1 0) * X = 3 copies (y_sign, y_hi) into K3(5 4 3) * X = 6 copies (z_sign, z_hi) into K3(8 7 6).GINF TXA \ Set Y = X * 2 ASL A TAY LDA UNIV,Y \ Get the high byte of the address of the X-th ship STA INF \ from UNIV and store it in INF LDA UNIV+1,Y \ Get the low byte of the address of the X-th ship STA INF+1 \ from UNIV and store it in INF RTS \ Return from the subroutineName: GINF [View individually] Type: Subroutine Category: Universe Summary: Fetch the address of a ship's data block into INF

Get the address of the data block for ship slot X and store it in INF. This address is fetched from the UNIV table, which stores the addresses of the 13 ship data blocks in workspace K%. Arguments: X The ship slot number for which we want the data block address.NWSPS JSR SPBLB \ Light up the space station bulb on the dashboard LDX #1 \ Set the AI flag in byte #32 to 1 (friendly, no AI, has STX INWK+32 \ E.C.M.) DEX \ Set pitch counter to 0 (no pitch, roll only) STX INWK+30 \STX INWK+31 \ This instruction is commented out in the original \ source. It would set the exploding state and missile \ count to 0 STX FRIN+1 \ Set the sun/space station slot at FRIN+1 to 0, to \ indicate we should show the space station rather than \ the sun DEX \ Set roll counter to 255 (maximum roll with no STX INWK+29 \ damping) LDX #10 \ Call NwS1 to flip the sign of nosev_x_hi (byte #10) JSR NwS1 JSR NwS1 \ And again to flip the sign of nosev_y_hi (byte #12) JSR NwS1 \ And again to flip the sign of nosev_z_hi (byte #14) LDA #LO(LSO) \ Set bytes #33 and #34 to point to LSO for the ship STA INWK+33 \ line heap for the space station LDA #HI(LSO) STA INWK+34 LDA #SST \ Set A to the space station type, and fall through \ into NWSHP to finish adding the space station to the \ universeName: NWSPS [View individually] Type: Subroutine Category: Universe Summary: Add a new space station to our local bubble of universe.NWSHP STA T \ Store the ship type in location T LDX #0 \ Before we can add a new ship, we need to check \ whether we have an empty slot we can put it in. To do \ this, we need to loop through all the slots to look \ for an empty one, so set a counter in X that starts \ from the first slot at 0. When ships are killed, then \ the slots are shuffled down by the KILLSHP routine, so \ the first empty slot will always come after the last \ filled slot. This allows us to tack the new ship's \ data block and ship line heap onto the end of the \ existing ship data and heap, as shown in the memory \ map below .NWL1 LDA FRIN,X \ Load the ship type for the X-th slot BEQ NW1 \ If it is zero, then this slot is empty and we can use \ it for our new ship, so jump down to NW1 INX \ Otherwise increment X to point to the next slot CPX #NOSH \ If we haven't reached the last slot yet, loop back up BCC NWL1 \ to NWL1 to check the next slot .NW3 CLC \ Otherwise we don't have an empty slot, so we can't RTS \ add a new ship, so clear the C flag to indicate that \ we have not managed to create the new ship, and return \ from the subroutine .NW1 \ If we get here, then we have found an empty slot at \ index X, so we can go ahead and create our new ship. \ We do that by creating a ship data block at INWK and, \ when we are done, copying the block from INWK into \ the K% workspace (specifically, to INF) JSR GINF \ Get the address of the data block for ship slot X \ (which is in workspace K%) and store it in INF LDA T \ If the type of ship that we want to create is BMI NW2 \ negative, then this indicates a planet or sun, so \ jump down to NW2, as the next section sets up a ship \ data block, which doesn't apply to planets and suns, \ as they don't have things like shields, missiles, \ vertices and edges \ This is a ship, so first we need to set up various \ pointers to the ship blueprint we will need. The \ blueprints for each ship type in Elite are stored \ in a table at location XX21, so refer to the comments \ on that variable for more details on the data we're \ about to access ASL A \ Set Y = ship type * 2 TAY LDA XX21-2,Y \ The ship blueprints at XX21 start with a lookup STA XX0 \ table that points to the individual ship blueprints, \ so this fetches the low byte of this particular ship \ type's blueprint and stores it in XX0 LDA XX21-1,Y \ Fetch the high byte of this particular ship type's STA XX0+1 \ blueprint and store it in XX0+1 CPY #2*SST \ If the ship type is a space station (SST), then jump BEQ NW6 \ to NW6, skipping the heap space steps below, as the \ space station has its own line heap at LSO (which it \ shares with the sun) \ We now want to allocate space for a heap that we can \ use to store the lines we draw for our new ship (so it \ can easily be erased from the screen again). SLSP \ points to the start of the current heap space, and we \ can extend it downwards with the heap for our new ship \ (as the heap space always ends just before the WP \ workspace) LDY #5 \ Fetch ship blueprint byte #5, which contains the LDA (XX0),Y \ maximum heap size required for plotting the new ship, STA T1 \ and store it in T1 LDA SLSP \ Take the 16-bit address in SLSP and subtract T1, SEC \ storing the 16-bit result in INWK(34 33), so this now SBC T1 \ points to the start of the line heap for our new ship STA INWK+33 LDA SLSP+1 SBC #0 STA INWK+34 \ We now need to check that there is enough free space \ for both this new line heap and the new data block \ for our ship. In memory, this is the layout of the \ ship data blocks and ship line heaps: \ \ +-----------------------------------+ &0F34 \ | | \ | WP workspace | \ | | \ +-----------------------------------+ &0D40 = WP \ | | \ | Current ship line heap | \ | | \ +-----------------------------------+ SLSP \ | | \ | Proposed heap for new ship | \ | | \ +-----------------------------------+ INWK(34 33) \ | | \ . . \ . . \ . . \ . . \ . . \ | | \ +-----------------------------------+ INF + NI% \ | | \ | Proposed data block for new ship | \ | | \ +-----------------------------------+ INF \ | | \ | Existing ship data blocks | \ | | \ +-----------------------------------+ &0900 = K% \ \ So, to work out if we have enough space, we have to \ make sure there is room between the end of our new \ ship data block at INF + NI%, and the start of the \ proposed heap for our new ship at the address we \ stored in INWK(34 33). Or, to put it another way, we \ and to make sure that: \ \ INWK(34 33) > INF + NI% \ \ which is the same as saying: \ \ INWK+33 - INF > NI% \ \ because INWK is in zero page, so INWK+34 = 0 LDA INWK+33 \ Calculate INWK+33 - INF, again using 16-bit \SEC \ arithmetic, and put the result in (A Y), so the high SBC INF \ byte is in A and the low byte in Y. The SEC TAY \ instruction is commented out in the original source; LDA INWK+34 \ as the previous subtraction will never underflow, it SBC INF+1 \ is superfluous BCC NW3+1 \ If we have an underflow from the subtraction, then \ INF > INWK+33 and we definitely don't have enough \ room for this ship, so jump to NW3+1, which clears \ the C flag and returns from the subroutine BNE NW4 \ If the subtraction of the high bytes in A is not \ zero, and we don't have underflow, then we definitely \ have enough space, so jump to NW4 to continue setting \ up the new ship CPY #NI% \ Otherwise the high bytes are the same in our BCC NW3+1 \ subtraction, so now we compare the low byte of the \ result (which is in Y) with NI%. This is the same as \ doing INWK+33 - INF > NI% (see above). If this isn't \ true, the C flag will be clear and we don't have \ enough space, so we jump to NW3+1, which clears the \ C flag and returns from the subroutine .NW4 LDA INWK+33 \ If we get here then we do have enough space for our STA SLSP \ new ship, so store the new bottom of the ship line LDA INWK+34 \ heap (i.e. INWK+33) in SLSP, doing both the high and STA SLSP+1 \ low bytes .NW6 LDY #14 \ Fetch ship blueprint byte #14, which contains the LDA (XX0),Y \ ship's energy, and store it in byte #35 STA INWK+35 LDY #19 \ Fetch ship blueprint byte #19, which contains the LDA (XX0),Y \ number of missiles and laser power, and AND with %111 AND #%00000111 \ to extract the number of missiles before storing in STA INWK+31 \ byte #31 LDA T \ Restore the ship type we stored above .NW2 STA FRIN,X \ Store the ship type in the X-th byte of FRIN, so the \ this slot is now shown as occupied in the index table TAX \ Copy the ship type into X BMI P%+5 \ If the ship type is negative (planet or sun), then \ skip the following instruction INC MANY,X \ Increment the total number of ships of type X LDY #(NI%-1) \ The final step is to copy the new ship's data block \ from INWK to INF, so set up a counter for NI% bytes \ in Y .NWL3 LDA INWK,Y \ Load the Y-th byte of INWK and store in the Y-th byte STA (INF),Y \ of the workspace pointed to by INF DEY \ Decrement the loop counter BPL NWL3 \ Loop back for the next byte until we have copied them \ all over SEC \ We have successfully created our new ship, so set the \ C flag to indicate success RTS \ Return from the subroutineName: NWSHP [View individually] Type: Subroutine Category: Universe Summary: Add a new ship to our local bubble of universe

This creates a new block of ship data in the K% workspace, allocates a new block in the ship line heap at WP, adds the new ship's type into the first empty slot in FRIN, and adds a pointer to the ship data into UNIV. If there isn't enough free memory for the new ship, it isn't added. Arguments: A The type of the ship to add (see variable XX21 for a list of ship types) Returns: C flag Set if the ship was successfully added, clear if it wasn't (as there wasn't enough free memory).NwS1 LDA INWK,X \ Load the X-th byte of INWK into A and flip bit 7, EOR #%10000000 \ storing the result back in the X-th byte of INWK STA INWK,X INX \ Add 2 to X INX RTS \ Return from the subroutineName: NwS1 [View individually] Type: Subroutine Category: Universe Summary: Flip the sign and double an INWK byte

Flip the sign of the INWK byte at offset X, and increment X by 2. This is used by the space station creation routine at NWSPS. Arguments: X The offset of the INWK byte to be flipped Returns: X X is incremented by 2.ABORT LDX #&FF \ Set X to &FF, which is the value of MSTG when we have \ no target lock for our missile \ Fall through into ABORT2 to set the missile lock to \ the value in X, which effectively disarms the missileName: ABORT [View individually] Type: Subroutine Category: Dashboard Summary: Disarm missiles and update the dashboard indicators.ABORT2 STX MSTG \ Store the target of our missile lock in MSTG LDX NOMSL \ Call MSBAR to update the leftmost indicator in the JSR MSBAR \ dashboard's missile bar, which returns with Y = 0 STY MSAR \ Set MSAR = 0 to indicate that the leftmost missile \ is no longer seeking a target lock RTS \ Return from the subroutineName: ABORT2 [View individually] Type: Subroutine Category: Dashboard Summary: Set/unset the lock target for a missile and update the dashboard

Set the lock target for the leftmost missile and update the dashboard. Arguments: X The slot number of the ship to lock our missile onto, or &FF to remove missile lock Y The new colour of the missile indicator: * &00 = black (no missile) * &0E = red (armed and locked) * &E0 = yellow/white (armed) * &EE = green/cyan (disarmed).ECBLB2 LDA #32 \ Set the E.C.M. countdown timer in ECMA to 32 STA ECMA ASL A \ Call the NOISE routine with A = 64 to make the sound JSR NOISE \ of the E.C.M. being switched on \ Fall through into ECBLB to light up the E.C.M. bulbName: ECBLB2 [View individually] Type: Subroutine Category: Dashboard Summary: Start up the E.C.M. (indicator, start countdown and make sound)

Light up the E.C.M. indicator bulb on the dashboard, set the E.C.M. countdown timer to 32, and start making the E.C.M. sound..ECBLB LDA #7*8 \ The E.C.M. bulb is in character block number 7 \ with each character taking 8 bytes, so this sets the \ low byte of the screen address of the character block \ we want to draw to LDX #LO(ECBT) \ Set (Y X) to point to the character definition in LDY #HI(ECBT) \ ECBT. The LDY has no effect, as we overwrite Y with \ the jump to BULB-2, which writes the high byte of SPBT \ into Y. This works as long as ECBT and SPBT are in \ the same page of memory, so perhaps the BNE below got \ changed from BULB to BULB-2 so they could remove the \ LDY, but for some reason it didn't get culled? Who \ knows... BNE BULB-2 \ Jump down to BULB-2 (this BNE is effectively a JMP as \ A will never be zero)Name: ECBLB [View individually] Type: Subroutine Category: Dashboard Summary: Light up the E.C.M. indicator bulb ("E") on the dashboard

This draws (or erases) the E.C.M. indicator bulb ("E") on the dashboard..SPBLB LDA #24*8 \ The space station bulb is in character block number 24 \ with each character taking 8 bytes, so this sets the \ low byte of the screen address of the character block \ we want to draw to LDX #LO(SPBT) \ Set (Y X) to point to the character definition in SPBT LDY #HI(SPBT)Name: SPBLB [View individually] Type: Subroutine Category: Dashboard Summary: Light up the space station indicator ("S") on the dashboard

This draws (or erases) the space station indicator bulb ("S") on the dashboard. Other entry points: BULB-2 Set the Y screen address.BULB STA SC \ Store the low byte of the screen address in SC STX P+1 \ Set P(2 1) = (Y X) STY P+2 LDA #&7D \ Set A to the high byte of the screen address, which is \ &7D as the bulbs are both in the character row from \ &7D00 to &7DFF JMP RREN \ Call RREN to print the character definition pointed to \ by P(2 1) at the screen address pointed to by (A SC), \ returning from the subroutine using a tail callName: BULB [View individually] Type: Subroutine Category: Dashboard Summary: Draw an indicator bulb on the dashboard

Arguments: A The y-coordinate of the bulb as a low-byte screen address offset within screen page &7D (as both bulbs are on this character row in the dashboard) (Y X) The address of the character definition of the bulb to be drawn (i.e. ECBT for the E.C.M. bulb, or SPBT for the space station bulb).ECBT EQUB %11100000 EQUB %11100000 EQUB %10000000Name: ECBT [View individually] Type: Variable Category: Dashboard Summary: The character definition for the E.C.M. indicator

The character definition for the E.C.M. indicator's "E" bulb that gets displayed on the dashboard. The E.C.M. indicator uses the first 5 rows of the space station's "S" bulb below, as the bottom 5 rows of the "E" match the top 5 rows of the "S". Each pixel is in mode 5 colour 2 (%10), which is yellow/white..SPBT EQUB %11100000 EQUB %11100000 EQUB %10000000 EQUB %11100000 EQUB %11100000 EQUB %00100000 EQUB %11100000 EQUB %11100000Name: SPBT [View individually] Type: Variable Category: Dashboard Summary: The character definition for the space station indicator

The character definition for the space station indicator's "S" bulb that gets displayed on the dashboard. Each pixel is in mode 5 colour 2 (%10), which is yellow/white..MSBAR TXA \ Set T = X * 8 ASL A ASL A ASL A STA T LDA #49 \ Set SC = 49 - T SBC T \ = 48 + 1 - (X * 8) STA SC \ So the low byte of SC(1 0) contains the row address \ for the rightmost missile indicator, made up as \ follows: \ \ * 48 (character block 7, or byte #7 * 8 = 48, which \ is the character block of the rightmost missile \ \ * 1 (so we start drawing on the second row of the \ character block) \ \ * Move right one character (8 bytes) for each count \ of X, so when X = 0 we are drawing the rightmost \ missile, for X = 1 we hop to the left by one \ character, and so on LDA #&7E \ Set the high byte of SC(1 0) to &7E, the character row STA SCH \ that contains the missile indicators (i.e. the bottom \ row of the screen) TYA \ Set A to the correct colour, which is a 3-pixel wide \ mode 5 character row in the correct colour (for \ example, a green block has Y = &EE, or %11101110, so \ the missile blocks are 3 pixels wide, with the \ fourth pixel on the character row being empty) LDY #5 \ We now want to draw this line five times, so set a \ counter in Y .MBL1 STA (SC),Y \ Draw the 3-pixel row, and as we do not use EOR logic, \ this will overwrite anything that is already there \ (so drawing a black missile will delete what's there) DEY \ Decrement the counter for the next row BNE MBL1 \ Loop back to MBL1 if have more rows to draw RTS \ Return from the subroutineName: MSBAR [View individually] Type: Subroutine Category: Dashboard Summary: Draw a specific indicator in the dashboard's missile bar

Each indicator is a rectangle that's 3 pixels wide and 5 pixels high. If the indicator is set to black, this effectively removes a missile. Arguments: X The number of the missile indicator to update (counting from right to left, so indicator NOMSL is the leftmost indicator) Y The colour of the missile indicator: * &00 = black (no missile) * &0E = red (armed and locked) * &E0 = yellow/white (armed) * &EE = green/cyan (disarmed) Returns: Y Y is set to 0.PROJ LDA INWK \ Set P(1 0) = (x_hi x_lo) STA P \ = x LDA INWK+1 STA P+1 LDA INWK+2 \ Set A = x_sign JSR PLS6 \ Call PLS6 to calculate: \ \ (X K) = (A P) / (z_sign z_hi z_lo) \ = (x_sign x_hi x_lo) / (z_sign z_hi z_lo) \ = x / z BCS PL2-1 \ If the C flag is set then the result overflowed and \ the coordinate doesn't fit on the screen, so return \ from the subroutine with the C flag set (as PL2-1 \ contains an RTS) LDA K \ Set K3(1 0) = (X K) + #X ADC #X \ = #X + x / z STA K3 \ \ first doing the low bytes TXA \ And then the high bytes. #X is the x-coordinate of ADC #0 \ the centre of the mode 4 screen, so this converts the STA K3+1 \ space x-coordinate into a screen x-coordinate LDA INWK+3 \ Set P(1 0) = (y_hi y_lo) STA P LDA INWK+4 STA P+1 LDA INWK+5 \ Set A = -y_sign EOR #%10000000 JSR PLS6 \ Call PLS6 to calculate: \ \ (X K) = (A P) / (z_sign z_hi z_lo) \ = -(y_sign y_hi y_lo) / (z_sign z_hi z_lo) \ = -y / z BCS PL2-1 \ If the C flag is set then the result overflowed and \ the coordinate doesn't fit on the screen, so return \ from the subroutine with the C flag set (as PL2-1 \ contains an RTS) LDA K \ Set K4(1 0) = (X K) + #Y ADC #Y \ = #Y - y / z STA K4 \ \ first doing the low bytes TXA \ And then the high bytes. #Y is the y-coordinate of ADC #0 \ the centre of the mode 4 screen, so this converts the STA K4+1 \ space x-coordinate into a screen y-coordinate CLC \ Clear the C flag to indicate success RTS \ Return from the subroutineName: PROJ [View individually] Type: Subroutine Category: Drawing ships Summary: Project the current ship onto the screen Deep dive: Extended screen coordinates

Project the current ship's location onto the screen, either returning the screen coordinates of the projection (if it's on-screen), or returning an error via the C flag. In this context, "on-screen" means that the point is projected into the following range: centre of screen - 1024 < x < centre of screen + 1024 centre of screen - 1024 < y < centre of screen + 1024 This is to cater for ships (and, more likely, planets and suns) whose centres are off-screen but whose edges may still be visible. The projection calculation is: K3(1 0) = #X + x / z K4(1 0) = #Y + y / z where #X and #Y are the pixel x-coordinate and y-coordinate of the centre of the screen. Arguments: INWK The ship data block for the ship to project on-screen Returns: K3(1 0) The x-coordinate of the ship's projection on-screen K4(1 0) The y-coordinate of the ship's projection on-screen C flag Set if the ship's projection doesn't fit on the screen, clear if it does project onto the screen A Contains K4+1, the high byte of the y-coordinate.PL2 LDA TYPE \ Shift bit 0 of the planet/sun's type into the C flag LSR A BCS P%+5 \ If the planet/sun's type has bit 0 clear, then it's \ either 128 or 130, which is a planet; meanwhile, the \ sun has type 129, which has bit 0 set. So if this is \ the sun, skip the following instruction JMP WPLS2 \ This is the planet, so jump to WPLS2 to remove it from \ screen, returning from the subroutine using a tail \ call JMP WPLS \ This is the sun, so jump to WPLS to remove it from \ screen, returning from the subroutine using a tail \ callName: PL2 [View individually] Type: Subroutine Category: Drawing planets Summary: Remove the planet or sun from the screen

Other entry points: PL2-1 Contains an RTS.PLANET LDA INWK+8 \ Set A = z_sign (the highest byte in the planet/sun's \ coordinates) BMI PL2 \ If A is negative then the planet/sun is behind us, so \ jump to PL2 to remove it from the screen, returning \ from the subroutine using a tail call CMP #48 \ If A >= 48 then the planet/sun is too far away to be BCS PL2 \ seen, so jump to PL2 to remove it from the screen, \ returning from the subroutine using a tail call ORA INWK+7 \ Set A to z_sign OR z_hi to get the maximum of the two BEQ PL2 \ If the maximum is 0, then the planet/sun is too close \ to be shown, so jump to PL2 to remove it from the \ screen, returning from the subroutine using a tail \ call JSR PROJ \ Project the planet/sun onto the screen, returning the \ centre's coordinates in K3(1 0) and K4(1 0) BCS PL2 \ If the C flag is set by PROJ then the planet/sun is \ not visible on-screen, so jump to PL2 to remove it \ from the screen, returning from the subroutine using \ a tail call LDA #96 \ Set (A P+1 P) = (0 96 0) = 24576 STA P+1 \ LDA #0 \ This represents the planet/sun's radius at a distance STA P \ of z = 1 JSR DVID3B2 \ Call DVID3B2 to calculate: \ \ K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo) \ = (0 96 0) / z \ = 24576 / z \ \ so K now contains the planet/sun's radius, reduced by \ the actual distance to the planet/sun. We know that \ K+3 and K+2 will be 0, as the number we are dividing, \ (0 96 0), fits into the two bottom bytes, so the \ result is actually in K(1 0) LDA K+1 \ If the high byte of the reduced radius is zero, jump BEQ PL82 \ to PL82, as K contains the radius on its own LDA #248 \ Otherwise set K = 248, to use as our one-byte radius STA K .PL82 LDA TYPE \ If the planet/sun's type has bit 0 clear, then it's LSR A \ either 128 or 130, which is a planet (the sun has type BCC PL9 \ 129, which has bit 0 set). So jump to PL9 to draw the \ planet with radius K, returning from the subroutine \ using a tail call JMP SUN \ Otherwise jump to SUN to draw the sun with radius K, \ returning from the subroutine using a tail callName: PLANET [View individually] Type: Subroutine Category: Drawing planets Summary: Draw the planet or sun

Arguments: INWK The planet or sun's ship data block.PL9 JSR WPLS2 \ Call WPLS2 to remove the planet from the screen JSR CIRCLE \ Call CIRCLE to draw the planet's new circle BCS PL20 \ If the call to CIRCLE returned with the C flag set, \ then the circle does not fit on-screen, so jump to \ PL20 to return from the subroutine LDA K+1 \ If K+1 is zero, jump to PL25 as K(1 0) < 256, so the BEQ PL25 \ planet fits on the screen .PL20 RTS \ The planet doesn't fit on-screen, so return from the \ subroutine .PL25 LDA TYPE \ If the planet type is 128 then it has an equator and CMP #128 \ a meridian, so this jumps to PL26 if this is not a BNE PL26 \ planet with an equator - in other words, if it is a \ planet with a crater \ Otherwise this is a planet with an equator and \ meridian, so fall through into the following to draw \ themName: PL9 (Part 1 of 3) [View individually] Type: Subroutine Category: Drawing planets Summary: Draw the planet, with either an equator and meridian, or a crater

Draw the planet with radius K at pixel coordinate (K3, K4), and with either an equator and meridian, or a crater. Arguments: K(1 0) The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet INWK The planet's ship data blockLDA K \ If the planet's radius is less than 6, the planet is CMP #6 \ too small to show a crater, so jump to PL20 to return BCC PL20 \ from the subroutine LDA INWK+14 \ Set P = -nosev_z_hi EOR #%10000000 STA P LDA INWK+20 \ Set A = roofv_z_hi JSR PLS4 \ Call PLS4 to calculate the following: \ \ CNT2 = arctan(P / A) / 4 \ = arctan(-nosev_z_hi / roofv_z_hi) / 4 \ \ and give the result the opposite sign to nosev_z_hi LDX #9 \ Set X to 9 so the call to PLS1 divides nosev_x JSR PLS1 \ Call PLS1 to calculate the following: STA K2 \ STY XX16 \ (XX16 K2) = nosev_x / z \ \ and increment X to point to nosev_y for the next call JSR PLS1 \ Call PLS1 to calculate the following: STA K2+1 \ STY XX16+1 \ (XX16+1 K2+1) = nosev_y / z LDX #15 \ Set X to 15 so the call to PLS5 divides roofv_x JSR PLS5 \ Call PLS5 to calculate the following: \ \ (XX16+2 K2+2) = roofv_x / z \ \ (XX16+3 K2+3) = roofv_y / z JSR PLS2 \ Call PLS2 to draw the first meridian LDA INWK+14 \ Set P = -nosev_z_hi EOR #%10000000 STA P LDA INWK+26 \ Set A = sidev_z_hi, so the second meridian will be at \ 90 degrees to the first JSR PLS4 \ Call PLS4 to calculate the following: \ \ CNT2 = arctan(P / A) / 4 \ = arctan(-nosev_z_hi / sidev_z_hi) / 4 \ \ and give the result the opposite sign to nosev_z_hi LDX #21 \ Set X to 21 so the call to PLS5 divides sidev_x JSR PLS5 \ Call PLS5 to calculate the following: \ \ (XX16+2 K2+2) = sidev_x / z \ \ (XX16+3 K2+3) = sidev_y / z JMP PLS2 \ Jump to PLS2 to draw the second meridian, returning \ from the subroutine with a tail callName: PL9 (Part 2 of 3) [View individually] Type: Subroutine Category: Drawing planets Summary: Draw the planet's equator and meridian Deep dive: Drawing meridians and equators

Draw the planet's equator and meridian. Arguments: K(1 0) The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet INWK The planet's ship data block.PL26 LDA INWK+20 \ Set A = roofv_z_hi BMI PL20 \ If A is negative, the crater is on the far side of the \ planet, so return from the subroutine (as PL2 \ contains an RTS) LDX #15 \ Set X = 15, so the following call to PLS3 operates on \ roofv JSR PLS3 \ Call PLS3 to calculate: \ \ (Y A P) = 222 * roofv_x / z \ \ to give the x-coordinate of the crater offset and \ increment X to point to roofv_y for the next call CLC \ Calculate: ADC K3 \ STA K3 \ K3(1 0) = (Y A) + K3(1 0) \ = 222 * roofv_x / z + x-coordinate of planet \ centre \ \ starting with the high bytes TYA \ And then doing the low bytes, so now K3(1 0) contains ADC K3+1 \ the x-coordinate of the crater offset plus the planet STA K3+1 \ centre to give the x-coordinate of the crater's centre JSR PLS3 \ Call PLS3 to calculate: \ \ (Y A P) = 222 * roofv_y / z \ \ to give the y-coordinate of the crater offset STA P \ Calculate: LDA K4 \ SEC \ K4(1 0) = K4(1 0) - (Y A) SBC P \ = 222 * roofv_x / z - y-coordinate of planet STA K4 \ centre \ \ starting with the low bytes STY P \ And then doing the low bytes, so now K4(1 0) contains LDA K4+1 \ the y-coordinate of the crater offset plus the planet SBC P \ centre to give the y-coordinate of the crater's centre STA K4+1 LDX #9 \ Set X = 9, so the following call to PLS1 operates on \ nosev JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = nosev_x / z \ \ and increment X to point to nosev_y for the next call LSR A \ Set (XX16 K2) = (Y A) / 2 STA K2 STY XX16 JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = nosev_y / z \ \ and increment X to point to nosev_z for the next call LSR A \ Set (XX16+1 K2+1) = (Y A) / 2 STA K2+1 STY XX16+1 LDX #21 \ Set X = 21, so the following call to PLS1 operates on \ sidev JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = sidev_x / z \ \ and increment X to point to sidev_y for the next call LSR A \ Set (XX16+2 K2+2) = (Y A) / 2 STA K2+2 STY XX16+2 JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = sidev_y / z \ \ and increment X to point to sidev_z for the next call LSR A \ Set (XX16+3 K2+3) = (Y A) / 2 STA K2+3 STY XX16+3 LDA #64 \ Set TGT = 64, so we draw a full circle in the call to STA TGT \ PLS22 below LDA #0 \ Set CNT2 = 0 as we are drawing a full circle, so we STA CNT2 \ don't need to apply an offset JMP PLS22 \ Jump to PLS22 to draw the crater, returning from the \ subroutine with a tail callName: PL9 (Part 3 of 3) [View individually] Type: Subroutine Category: Drawing planets Summary: Draw the planet's crater Deep dive: Drawing craters

Draw the planet's crater. Arguments: K(1 0) The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet INWK The planet's ship data block.PLS1 LDA INWK,X \ Set P = nosev_x_lo STA P LDA INWK+1,X \ Set P+1 = |nosev_x_hi| AND #%01111111 STA P+1 LDA INWK+1,X \ Set A = sign bit of nosev_x_lo AND #%10000000 JSR DVID3B2 \ Call DVID3B2 to calculate: \ \ K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo) LDA K \ Fetch the lowest byte of the result into A LDY K+1 \ Fetch the second byte of the result into Y BEQ P%+4 \ If the second byte is 0, skip the next instruction LDA #254 \ The second byte is non-zero, so the result won't fit \ into one byte, so set A = 254 as our maximum one-byte \ value to return LDY K+3 \ Fetch the sign of the result from K+3 into Y INX \ Add 2 to X so the index points to the next coordinate INX \ in this orientation vector (so consecutive calls to \ the routine will start with x, then move onto y and z) RTS \ Return from the subroutineName: PLS1 [View individually] Type: Subroutine Category: Drawing planets Summary: Calculate (Y A) = nosev_x / z

Calculate the following division of a specified value from one of the orientation vectors (in this example, nosev_x): (Y A) = nosev_x / z where z is the z-coordinate of the planet from INWK. The result is an 8-bit magnitude in A, with maximum value 254, and just a sign bit (bit 7) in Y. Arguments: X Determines which of the INWK orientation vectors to divide: * X = 9, 11, 13: divides nosev_x, nosev_y, nosev_z * X = 15, 17, 19: divides roofv_x, roofv_y, roofv_z * X = 21, 23, 25: divides sidev_x, sidev_y, sidev_z INWK The planet's ship data block Returns: A The result as an 8-bit magnitude with maximum value 254 Y The sign of the result in bit 7 K+3 Also the sign of the result in bit 7 X X gets incremented by 2 so it points to the next coordinate in this orientation vector (so consecutive calls to the routine will start with x, then move onto y and then z).PLS2 LDA #31 \ Set TGT = 31, so we only draw half a circle STA TGT \ Fall through into PLS22 to draw the half circleName: PLS2 [View individually] Type: Subroutine Category: Drawing planets Summary: Draw a half-circle

Draw a half-circle, used for the planet's equator and meridian..PLS22 LDX #0 \ Set CNT = 0 STX CNT DEX \ Set FLAG = &FF to reset the ball line heap in the call STX FLAG \ to the BLINE routine below .PLL4 LDA CNT2 \ Set A = CNT2 mod 32 AND #31 TAX LDA SNE,X \ Set Q = sin(CNT2) STA Q LDA K2+2 \ Set A = K2+2 \ = |roofv_x / z| JSR FMLTU \ Set R = A * Q / 256 STA R \ = |roofv_x / z| * sin(CNT2) / 256 LDA K2+3 \ Set A = K2+2 \ = |roofv_y / z| JSR FMLTU \ Set K = A * Q / 256 STA K \ = |roofv_y / z| * sin(CNT2) / 256 LDX CNT2 \ If CNT2 >= 33 then this sets the C flag, otherwise CPX #33 \ it's clear LDA #0 \ Shift the C flag into the sign bit of XX16+5, so: ROR A \ STA XX16+5 \ XX16+5 = +ve if CNT2 < 33 \ -ve if CNT2 >= 33 LDA CNT2 \ Set A = (CNT2 + 16) mod 32 CLC ADC #16 AND #31 TAX LDA SNE,X \ Set Q = sin(CNT2 + 16) STA Q \ = cos(CNT2) LDA K2+1 \ Set A = K2+1 \ = |nosev_y / z| JSR FMLTU \ Set K+2 = A * Q / 256 STA K+2 \ = |nosev_y / z| * cos(CNT2) / 256 LDA K2 \ Set A = K2 \ = |nosev_x / z| JSR FMLTU \ Set P = A * Q / 256 STA P \ = |nosev_x / z| * cos(CNT2) / 256 LDA CNT2 \ If (CNT2 + 15) mod 64 >= 33 then this sets the C flag, ADC #15 \ otherwise it's clear AND #63 CMP #33 LDA #0 \ Shift the C flag into the sign bit of XX16+4, so: ROR A \ STA XX16+4 \ XX16+4 = +ve if (CNT2 + 15) mod 64 < 33, \ -ve if (CNT2 + 15) mod 64 >= 33 LDA XX16+5 \ Set S = the sign of (roofv_x / z * CNT2 < 33 sign) EOR XX16+2 STA S LDA XX16+4 \ Set A = the sign of (nosev_x / z * CNT2 + 15 < 33 EOR XX16 \ sign) JSR ADD \ Set (A X) = (A P) + (S R) \ = (nosev_x / z) * cos(CNT2) \ + (roofv_x / z) * sin(CNT2) STA T \ Store the high byte in T, so the result is now: \ \ (T X) = (nosev_x / z) * cos(CNT2) \ + (roofv_x / z) * sin(CNT2) BPL PL42 \ If the result is positive, jump down to PL42 TXA \ The result is negative, so we need to negate the EOR #%11111111 \ magnitude using two's complement, first doing the low CLC \ byte in X ADC #1 TAX LDA T \ And then the high byte in T, making sure to leave the EOR #%01111111 \ sign bit alone ADC #0 STA T .PL42 TXA \ Set K6(1 0) = K3(1 0) + (T X) ADC K3 \ STA K6 \ starting with the low bytes LDA T \ And then doing the high bytes, so we now get: ADC K3+1 \ STA K6+1 \ K6(1 0) = K3(1 0) + (nosev_x / z) * cos(CNT2) \ + (roofv_x / z) * sin(CNT2) LDA K \ Set R = K = |roofv_y / z| * sin(CNT2) / 256 STA R LDA XX16+5 \ Set S = the sign of (roofv_y / z * CNT2 < 33 sign) EOR XX16+3 STA S LDA K+2 \ Set P = K+2 = |nosev_y / z| * cos(CNT2) / 256 STA P LDA XX16+4 \ Set A = the sign of (nosev_y / z * CNT2 + 15 < 33 EOR XX16+1 \ sign) JSR ADD \ Set (A X) = (A P) + (S R) \ = |nosev_y / z| * cos(CNT2) \ + |roofv_y / z| * sin(CNT2) EOR #%10000000 \ Store the negated high byte in T, so the result is STA T \ now: \ \ (T X) = - |nosev_y / z| * cos(CNT2) \ - |roofv_y / z| * sin(CNT2) BPL PL43 \ If the result is positive, jump down to PL43 TXA \ The result is negative, so we need to negate the EOR #%11111111 \ magnitude using two's complement, first doing the low CLC \ byte in X ADC #1 TAX LDA T \ And then the high byte in T, making sure to leave the EOR #%01111111 \ sign bit alone ADC #0 STA T .PL43 JSR BLINE \ Call BLINE to draw this segment, which also returns \ the updated value of CNT in A CMP TGT \ If CNT > TGT then jump to PL40 to stop drawing the BEQ P%+4 \ circle (which is how we draw half-circles) BCS PL40 LDA CNT2 \ Set CNT2 = (CNT2 + STP) mod 64 CLC ADC STP AND #63 STA CNT2 JMP PLL4 \ Jump back to PLL4 to draw the next segment .PL40 RTS \ Return from the subroutineName: PLS22 [View individually] Type: Subroutine Category: Drawing planets Summary: Draw a circle or half-circle

Draw a circle or half-circle, used for the planet's equator and meridian, or crater. This routine is called from parts 2 and 3 of PL9, and does the following: K6(1 0) = K3(1 0) + (XX16 K2) * cos(CNT2) + (XX16+2 K2+2) * sin(CNT2) (T X) = - |XX16+1 K2+1| * cos(CNT2) - |XX16+3 K2+3| * sin(CNT2) before calling BLINE to draw a circle segment to these coordinates. Arguments: K(1 0) The planet's radius INWK The planet's ship data block TGT The number of segments to draw: * 32 for a half circle (a meridian) * 64 for a half circle (a crater) CNT2 The starting segment for drawing the half-circleJMP WPLS \ Jump to WPLS to remove the old sun from the screen. We \ only get here via the BCS just after the SUN entry \ point below, when there is no new sun to draw .PLF3 \ This is called from below to negate X and set A to \ &FF, for when the new sun's centre is off the bottom \ of the screen (so we don't need to draw its bottom \ half) TXA \ Negate X using two's complement, so X = ~X + 1 EOR #%11111111 \ CLC \ We do this because X is negative at this point, as it ADC #1 \ is calculated as 191 - the y-coordinate of the sun's TAX \ centre, and the centre is off the bottom of the \ screen, past 191. So we negate it to make it positive .PLF17 \ This is called from below to set A to &FF, for when \ the new sun's centre is right on the bottom of the \ screen (so we don't need to draw its bottom half) LDA #&FF \ Set A = &FF JMP PLF5 \ Jump to PLF5 .SUN LDA #1 \ Set LSX = 1 to indicate the sun line heap is about to STA LSX \ be filled up JSR CHKON \ Call CHKON to check whether any part of the new sun's \ circle appears on-screen, and of it does, set P(2 1) \ to the maximum y-coordinate of the new sun on-screen BCS PLF3-3 \ If CHKON set the C flag then the new sun's circle does \ not appear on-screen, so jump to WPLS (via the JMP at \ the top of this routine) to remove the sun from the \ screen, returning from the subroutine using a tail \ call LDA #0 \ Set A = 0 LDX K \ Set X = K = radius of the new sun CPX #96 \ If X >= 96, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 CPX #40 \ If X >= 40, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 CPX #16 \ If X >= 16, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 \ By now, A contains the following: \ \ * If radius is 96-255 then A = %111 = 7 \ \ * If radius is 40-95 then A = %11 = 3 \ \ * If radius is 16-39 then A = %1 = 1 \ \ * If radius is 0-15 then A = %0 = 0 \ \ The value of A determines the size of the new sun's \ ragged fringes - the bigger the sun, the bigger the \ fringes .PLF18 STA CNT \ Store the fringe size in CNT \ We now calculate the highest pixel y-coordinate of the \ new sun, given that P(2 1) contains the 16-bit maximum \ y-coordinate of the new sun on-screen LDA #2*Y-1 \ #Y is the y-coordinate of the centre of the mode 4 \ space view, so this sets Y to the y-coordinate of the \ bottom of the space view, i.e. 191 LDX P+2 \ If P+2 is non-zero, the maximum y-coordinate is off BNE PLF2 \ the bottom of the screen, so skip to PLF2 with A = 191 CMP P+1 \ If A < P+1, the maximum y-coordinate is underneath the BCC PLF2 \ the dashboard, so skip to PLF2 with A = 191 LDA P+1 \ Set A = P+1, the low byte of the maximum y-coordinate \ of the sun on-screen BNE PLF2 \ If A is non-zero, skip to PLF2 as it contains the \ value we are after LDA #1 \ Otherwise set A = 1, the top line of the screen .PLF2 STA TGT \ Set TGT to A, the maximum y-coordinate of the sun on \ screen \ We now calculate the number of lines we need to draw \ and the direction in which we need to draw them, both \ from the centre of the new sun LDA #2*Y-1 \ Set (A X) = y-coordinate of bottom of screen - K4(1 0) SEC \ SBC K4 \ Starting with the low bytes TAX LDA #0 \ And then doing the high bytes, so (A X) now contains SBC K4+1 \ the number of lines between the centre of the sun and \ the bottom of the screen. If it is positive then the \ centre of the sun is above the bottom of the screen, \ if it is negative then the centre of the sun is below \ the bottom of the screen BMI PLF3 \ If A < 0, then this means the new sun's centre is off \ the bottom of the screen, so jump up to PLF3 to negate \ the height in X (so it becomes positive), set A to &FF \ and jump down to PLF5 BNE PLF4 \ If A > 0, then the new sun's centre is at least a full \ screen above the bottom of the space view, so jump \ down to PLF4 to set X = radius and A = 0 INX \ Set the flags depending on the value of X DEX BEQ PLF17 \ If X = 0 (we already know A = 0 by this point) then \ jump up to PLF17 to set A to &FF before jumping down \ to PLF5 CPX K \ If X < the radius in K, jump down to PLF5, so if BCC PLF5 \ X >= the radius in K, we set X = radius and A = 0 .PLF4 LDX K \ Set X to the radius LDA #0 \ Set A = 0 .PLF5 STX V \ Store the height in V STA V+1 \ Store the direction in V+1 LDA K \ Set (A P) = K * K JSR SQUA2 STA K2+1 \ Set K2(1 0) = (A P) = K * K LDA P STA K2 \ By the time we get here, the variables should be set \ up as shown in the header for part 3 belowName: SUN (Part 1 of 4) [View individually] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Set up all the variables needed Deep dive: Drawing the sun

Draw a new sun with radius K at pixel coordinate (K3, K4), removing the old sun if there is one. This routine is used to draw the sun, as well as the star systems on the Short-range Chart. The first part sets up all the variables needed to draw the new sun. Arguments: K The new sun's radius K3(1 0) Pixel x-coordinate of the centre of the new sun K4(1 0) Pixel y-coordinate of the centre of the new sun SUNX(1 0) The x-coordinate of the vertical centre axis of the old sun (the one currently on-screen) Other entry points: RTS2 Contains an RTSLDY #2*Y-1 \ Set Y = y-coordinate of the bottom of the screen, \ which we use as a counter in the following routine to \ redraw the old sun LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the old sun that's currently LDA SUNX+1 \ on-screen STA YY+1 .PLFL2 CPY TGT \ If Y = TGT, we have reached the line where we will BEQ PLFL \ start drawing the new sun, so there is no need to \ keep erasing the old one, so jump down to PLFL LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the old sun's line on this \ line of the screen BEQ PLF13 \ If A = 0, skip the following call to HLOIN2 as there \ is no sun line on this line of the screen JSR HLOIN2 \ Call HLOIN2 to draw a horizontal line on pixel line Y, \ with centre point YY(1 0) and half-width A, and remove \ the line from the sun line heap once done .PLF13 DEY \ Decrement the loop counter BNE PLFL2 \ Loop back for the next line in the line heap until \ we have either gone through the entire heap, or \ reached the bottom row of the new sunName: SUN (Part 2 of 4) [View individually] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Start from bottom of screen and erase the old sun

This part erases the old sun, starting at the bottom of the screen and working upwards until we reach the bottom of the new sun..PLFL LDA V \ Set (T P) = V * V JSR SQUA2 \ = V^2 STA T LDA K2 \ Set (R Q) = K^2 - V^2 SEC \ SBC P \ First calculating the low bytes STA Q LDA K2+1 \ And then doing the high bytes SBC T STA R STY Y1 \ Store Y in Y1, so we can restore it after the call to \ LL5 JSR LL5 \ Set Q = SQRT(R Q) \ = SQRT(K^2 - V^2) \ \ So Q contains the half-width of the new sun's line at \ height V from the sun's centre - in other words, it \ contains the half-width of the sun's line on the \ current pixel row Y LDY Y1 \ Restore Y from Y1 JSR DORND \ Set A and X to random numbers AND CNT \ Reduce A to a random number in the range 0 to CNT, \ where CNT is the fringe size of the new sun CLC \ Set A = A + Q ADC Q \ \ So A now contains the half-width of the sun on row \ V, plus a random variation based on the fringe size BCC PLF44 \ If the above addition did not overflow, skip the \ following instruction LDA #255 \ The above overflowed, so set the value of A to 255 \ So A contains the half-width of the new sun on pixel \ line Y, changed by a random amount within the size of \ the sun's fringe .PLF44 LDX LSO,Y \ Set X to the line heap value for the old sun's line \ at row Y STA LSO,Y \ Store the half-width of the new row Y line in the line \ heap BEQ PLF11 \ If X = 0 then there was no sun line on pixel row Y, so \ jump to PLF11 LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the old sun that's currently LDA SUNX+1 \ on-screen STA YY+1 TXA \ Transfer the line heap value for the old sun's line \ from X into A JSR EDGES \ Call EDGES to calculate X1 and X2 for the horizontal \ line centred on YY(1 0) and with half-width A, i.e. \ the line for the old sun LDA X1 \ Store X1 and X2, the ends of the line for the old sun, STA XX \ in XX and XX+1 LDA X2 STA XX+1 LDA K3 \ Set YY(1 0) = K3(1 0), the x-coordinate of the centre STA YY \ of the new sun LDA K3+1 STA YY+1 LDA LSO,Y \ Fetch the half-width of the new row Y line from the \ line heap (which we stored above) JSR EDGES \ Call EDGES to calculate X1 and X2 for the horizontal \ line centred on YY(1 0) and with half-width A, i.e. \ the line for the new sun BCS PLF23 \ If the C flag is set, the new line doesn't fit on the \ screen, so jump to PLF23 to just draw the old line \ without drawing the new one \ At this point the old line is from XX to XX+1 and the \ new line is from X1 to X2, and both fit on-screen. We \ now want to remove the old line and draw the new one. \ We could do this by simply drawing the old one then \ drawing the new one, but instead Elite does this by \ drawing first from X1 to XX and then from X2 to XX+1, \ which you can see in action by looking at all the \ permutations below of the four points on the line and \ imagining what happens if you draw from X1 to XX and \ X2 to XX+1 using EOR logic. The six possible \ permutations are as follows, along with the result of \ drawing X1 to XX and then X2 to XX+1: \ \ X1 X2 XX____XX+1 -> +__+ + + \ \ X1 XX____X2____XX+1 -> +__+__+ + \ \ X1 XX____XX+1 X2 -> +__+__+__+ \ \ XX____X1____XX+1 X2 -> + +__+__+ \ \ XX____XX+1 X1 X2 -> + + +__+ \ \ XX____X1____X2____XX+1 -> + +__+ + \ \ They all end up with a line between X1 and Y1, which \ is what we want. There's probably a mathematical proof \ of why this works somewhere, but the above is probably \ easier to follow. \ \ We can draw from X1 to XX and X2 to XX+1 by swapping \ XX and X2 and drawing from X1 to X2, and then drawing \ from XX to XX+1, so let's do this now LDA X2 \ Swap XX and X2 LDX XX STX X2 STA XX JSR HLOIN \ Draw a horizontal line from (X1, Y1) to (X2, Y1) .PLF23 \ If we jump here from the BCS above when there is no \ new line this will just draw the old line LDA XX \ Set X1 = XX STA X1 LDA XX+1 \ Set X2 = XX+1 STA X2 .PLF16 JSR HLOIN \ Draw a horizontal line from (X1, Y1) to (X2, Y1) .PLF6 DEY \ Decrement the line number in Y to move to the line \ above BEQ PLF8 \ If we have reached the top of the screen, jump to PLF8 \ as we are done drawing (the top line of the screen is \ the border, so we don't draw there) LDA V+1 \ If V+1 is non-zero then we are doing the top half of BNE PLF10 \ the new sun, so jump down to PLF10 to increment V and \ decrease the width of the line we draw DEC V \ Decrement V, the height of the sun that we use to work \ out the width, so this makes the line get wider, as we \ move up towards the sun's centre BNE PLFL \ If V is non-zero, jump back up to PLFL to do the next \ screen line up DEC V+1 \ Otherwise V is 0 and we have reached the centre of the \ sun, so decrement V+1 to -1 so we start incrementing V \ each time, thus doing the top half of the new sun .PLFLS JMP PLFL \ Jump back up to PLFL to do the next screen line up .PLF11 \ If we get here then there is no old sun line on this \ line, so we can just draw the new sun's line. The new LDX K3 \ Set YY(1 0) = K3(1 0), the x-coordinate of the centre STX YY \ of the new sun's line LDX K3+1 STX YY+1 JSR EDGES \ Call EDGES to calculate X1 and X2 for the horizontal \ line centred on YY(1 0) and with half-width A, i.e. \ the line for the new sun BCC PLF16 \ If the line is on-screen, jump up to PLF16 to draw the \ line and loop round for the next line up LDA #0 \ The line is not on-screen, so set the line heap for STA LSO,Y \ line Y to 0, which means there is no sun line here BEQ PLF6 \ Jump up to PLF6 to loop round for the next line up \ (this BEQ is effectively a JMP as A is always zero) .PLF10 LDX V \ Increment V, the height of the sun that we use to work INX \ out the width, so this makes the line get narrower, as STX V \ we move up and away from the sun's centre CPX K \ If V <= the radius of the sun, we still have lines to BCC PLFLS \ draw, so jump up to PLFL (via PLFLS) to do the next BEQ PLFLS \ screen line upName: SUN (Part 3 of 4) [View individually] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Continue to move up the screen, drawing the new sun

This part draws the new sun. By the time we get to this point, the following variables should have been set up by parts 1 and 2: V As we draw lines for the new sun, V contains the vertical distance between the line we're drawing and the centre of the new sun. As we draw lines and move up the screen, we either decrement (bottom half) or increment (top half) this value. See the deep dive on "Drawing the sun" to see a diagram that shows V in action V+1 This determines which half of the new sun we are drawing as we work our way up the screen, line by line: * 0 means we are drawing the bottom half, so the lines get wider as we work our way up towards the centre, at which point we will move into the top half, and V+1 will switch to &FF * &FF means we are drawing the top half, so the lines get smaller as we work our way up, away from the centre TGT The maximum y-coordinate of the new sun on-screen (i.e. the screen y-coordinate of the bottom row of the new sun) CNT The fringe size of the new sun K2(1 0) The new sun's radius squared, i.e. K^2 Y The y-coordinate of the bottom row of the new sunLDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the old sun that's currently LDA SUNX+1 \ on-screen STA YY+1 .PLFL3 LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the old sun's line on this \ line of the screen BEQ PLF9 \ If A = 0, skip the following call to HLOIN2 as there \ is no sun line on this line of the screen JSR HLOIN2 \ Call HLOIN2 to draw a horizontal line on pixel line Y, \ with centre point YY(1 0) and half-width A, and remove \ the line from the sun line heap once done .PLF9 DEY \ Decrement the line number in Y to move to the line \ above BNE PLFL3 \ Jump up to PLFL3 to redraw the next line up, until we \ have reached the top of the screen .PLF8 \ If we get here, we have successfully made it from the \ bottom line of the screen to the top, and the old sun \ has been replaced by the new one CLC \ Clear the C flag to indicate success in drawing the \ sun LDA K3 \ Set SUNX(1 0) = K3(1 0) STA SUNX LDA K3+1 STA SUNX+1 .RTS2 RTS \ Return from the subroutineName: SUN (Part 4 of 4) [View individually] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Continue to the top of the screen, erasing old sun

This part erases any remaining traces of the old sun, now that we have drawn all the way to the top of the new sun..CIRCLE JSR CHKON \ Call CHKON to check whether the circle fits on-screen BCS RTS2 \ If CHKON set the C flag then the circle does not fit \ on-screen, so return from the subroutine (as RTS2 \ contains an RTS) LDA #0 \ Set LSX2 = 0 STA LSX2 LDX K \ Set X = K = radius LDA #8 \ Set A = 8 CPX #8 \ If the radius < 8, skip to PL89 BCC PL89 LSR A \ Halve A so A = 4 CPX #60 \ If the radius < 60, skip to PL89 BCC PL89 LSR A \ Halve A so A = 2 .PL89 STA STP \ Set STP = A. STP is the step size for the circle, so \ the above sets a smaller step size for bigger circles \ Fall through into CIRCLE2 to draw the circle with the \ correct step sizeName: CIRCLE [View individually] Type: Subroutine Category: Drawing circles Summary: Draw a circle for the planet

Draw a circle with the centre at (K3, K4) and radius K. Used to draw the planet's main outline. Arguments: K The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet.CIRCLE2 LDX #&FF \ Set FLAG = &FF to reset the ball line heap in the call STX FLAG \ to the BLINE routine below INX \ Set CNT = 0, our counter that goes up to 64, counting STX CNT \ segments in our circle .PLL3 LDA CNT \ Set A = CNT JSR FMLTU2 \ Call FMLTU2 to calculate: \ \ A = K * sin(A) \ = K * sin(CNT) LDX #0 \ Set T = 0, so we have the following: STX T \ \ (T A) = K * sin(CNT) \ \ which is the x-coordinate of the circle for this count LDX CNT \ If CNT < 33 then jump to PL37, as this is the right CPX #33 \ half of the circle and the sign of the x-coordinate is BCC PL37 \ correct EOR #%11111111 \ This is the left half of the circle, so we want to ADC #0 \ flip the sign of the x-coordinate in (T A) using two's TAX \ complement, so we start with the low byte and store it \ in X (the ADC adds 1 as we know the C flag is set) LDA #&FF \ And then we flip the high byte in T ADC #0 STA T TXA \ Finally, we restore the low byte from X, so we have \ now negated the x-coordinate in (T A) CLC \ Clear the C flag so we can do some more addition below .PL37 ADC K3 \ We now calculate the following: STA K6 \ \ K6(1 0) = (T A) + K3(1 0) \ \ to add the coordinates of the centre to our circle \ point, starting with the low bytes LDA K3+1 \ And then doing the high bytes, so we now have: ADC T \ STA K6+1 \ K6(1 0) = K * sin(CNT) + K3(1 0) \ \ which is the result we want for the x-coordinate LDA CNT \ Set A = CNT + 16 CLC ADC #16 JSR FMLTU2 \ Call FMLTU2 to calculate: \ \ A = K * sin(A) \ = K * sin(CNT + 16) \ = K * cos(CNT) TAX \ Set X = A \ = K * cos(CNT) LDA #0 \ Set T = 0, so we have the following: STA T \ \ (T X) = K * cos(CNT) \ \ which is the y-coordinate of the circle for this count LDA CNT \ Set A = (CNT + 15) mod 64 ADC #15 AND #63 CMP #33 \ If A < 33 (i.e. CNT is 0-16 or 48-64) then jump to BCC PL38 \ PL38, as this is the bottom half of the circle and the \ sign of the y-coordinate is correct TXA \ This is the top half of the circle, so we want to EOR #%11111111 \ flip the sign of the y-coordinate in (T X) using two's ADC #0 \ complement, so we start with the low byte in X (the TAX \ ADC adds 1 as we know the C flag is set) LDA #&FF \ And then we flip the high byte in T, so we have ADC #0 \ now negated the y-coordinate in (T X) STA T CLC \ Clear the C flag so we can do some more addition below .PL38 JSR BLINE \ Call BLINE to draw this segment, which also increases \ CNT by STP, the step size CMP #65 \ If CNT >=65 then skip the next instruction BCS P%+5 JMP PLL3 \ Jump back for the next segment CLC \ Clear the C flag to indicate success RTS \ Return from the subroutineName: CIRCLE2 [View individually] Type: Subroutine Category: Drawing circles Summary: Draw a circle (for the planet or chart) Deep dive: Drawing circles

Draw a circle with the centre at (K3, K4) and radius K. Used to draw the planet and the chart circles. Arguments: STP The step size for the circle K The circle's radius K3(1 0) Pixel x-coordinate of the centre of the circle K4(1 0) Pixel y-coordinate of the centre of the circle Returns: C flag The C flag is clear.WPLS2 LDY LSX2 \ If LSX2 is non-zero (which indicates the ball line BNE WP1 \ heap is empty), jump to WP1 to reset the line heap \ without redrawing the planet \ Otherwise Y is now 0, so we can use it as a counter to \ loop through the lines in the line heap, redrawing \ each one to remove the planet from the screen, before \ resetting the line heap once we are done .WPL1 CPY LSP \ If Y >= LSP then we have reached the end of the line BCS WP1 \ heap and have finished redrawing the planet (as LSP \ points to the end of the heap), so jump to WP1 to \ reset the line heap LDA LSY2,Y \ Set A to the y-coordinate of the current heap entry CMP #&FF \ If the y-coordinate is &FF, this indicates that the BEQ WP2 \ next point in the heap denotes the start of a line \ segment, so jump to WP2 to put it into (X1, Y1) STA Y2 \ Set (X2, Y2) to the x- and y-coordinates from the LDA LSX2,Y \ heap STA X2 JSR LOIN \ Draw a line from (X1, Y1) to (X2, Y2) INY \ Increment the loop counter to point to the next point LDA SWAP \ If SWAP is non-zero then we swapped the coordinates BNE WPL1 \ when filling the heap in BLINE, so loop back WPL1 \ for the next point in the heap LDA X2 \ Swap (X1, Y1) and (X2, Y2), so the next segment will STA X1 \ be drawn from the current (X2, Y2) to the next point LDA Y2 \ in the heap STA Y1 JMP WPL1 \ Loop back to WPL1 for the next point in the heap .WP2 INY \ Increment the loop counter to point to the next point LDA LSX2,Y \ Set (X1, Y1) to the x- and y-coordinates from the STA X1 \ heap LDA LSY2,Y STA Y1 INY \ Increment the loop counter to point to the next point JMP WPL1 \ Loop back to WPL1 for the next point in the heap .WP1 LDA #1 \ Set LSP = 1 to reset the ball line heap pointer STA LSP LDA #&FF \ Set LSX2 = &FF to indicate the ball line heap is empty STA LSX2 RTS \ Return from the subroutineName: WPLS2 [View individually] Type: Subroutine Category: Drawing planets Summary: Remove the planet from the screen

We do this by redrawing it using the lines stored in the ball line heap when the planet was originally drawn by the BLINE routine. Other entry points: WPLS-1 Contains an RTS.WPLS LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI WPLS-1 \ the subroutine (as WPLS-1 contains an RTS) LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the sun that's currently on LDA SUNX+1 \ screen STA YY+1 LDY #2*Y-1 \ #Y is the y-coordinate of the centre of the mode 4 \ space view, so this sets Y as a counter for the number \ of lines in the space view (i.e. 191), which is also \ the number of lines in the LSO block .WPL2 LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the sun's line on this line \ of the screen BEQ P%+5 \ If A = 0, skip the following call to HLOIN2 as there \ is no sun line on this line of the screen JSR HLOIN2 \ Call HLOIN2 to draw a horizontal line on pixel line Y, \ with centre point YY(1 0) and half-width A, and remove \ the line from the sun line heap once done DEY \ Decrement the loop counter BNE WPL2 \ Loop back for the next line in the line heap until \ we have gone through the entire heap DEY \ This sets Y to &FF, as we end the loop with Y = 0 STY LSX \ Set LSX to &FF to indicate the sun line heap is empty RTS \ Return from the subroutineName: WPLS [View individually] Type: Subroutine Category: Drawing suns Summary: Remove the sun from the screen

We do this by redrawing it using the lines stored in the sun line heap when the sun was originally drawn by the SUN routine. Arguments: SUNX(1 0) The x-coordinate of the vertical centre axis of the sun.EDGES STA T \ Set T to the line's half-length in argument A CLC \ We now calculate: ADC YY \ STA X2 \ (A X2) = YY(1 0) + A \ \ to set X2 to the x-coordinate of the right end of the \ line, starting with the low bytes LDA YY+1 \ And then adding the high bytes ADC #0 BMI ED1 \ If the addition is negative then the calculation has \ overflowed, so jump to ED1 to return a failure BEQ P%+6 \ If the high byte A from the result is 0, skip the \ next two instructions, as the result already fits on \ the screen LDA #254 \ The high byte is positive and non-zero, so we went STA X2 \ past the right edge of the screen, so clip X2 to the \ x-coordinate of the right edge of the screen LDA YY \ We now calculate: SEC \ SBC T \ (A X1) = YY(1 0) - argument A STA X1 \ \ to set X1 to the x-coordinate of the left end of the \ line, starting with the low bytes LDA YY+1 \ And then subtracting the high bytes SBC #0 BNE ED3 \ If the high byte subtraction is non-zero, then skip \ to ED3 CLC \ Otherwise the high byte of the subtraction was zero, \ so the line fits on-screen and we clear the C flag to \ indicate success RTS \ Return from the subroutine .ED3 BPL ED1 \ If the addition is positive then the calculation has \ underflowed, so jump to ED1 to return a failure LDA #2 \ The high byte is negative and non-zero, so we went STA X1 \ past the left edge of the screen, so clip X1 to the \ y-coordinate of the left edge of the screen CLC \ The line does fit on-screen, so clear the C flag to \ indicate success RTS \ Return from the subroutine .ED1 LDA #0 \ Set the Y-th byte of the LSO block to 0 STA LSO,Y SEC \ The line does not fit on the screen, so set the C flag \ to indicate this result RTS \ Return from the subroutineName: EDGES [View individually] Type: Subroutine Category: Drawing lines Summary: Draw a horizontal line given a centre and a half-width

Set X1 and X2 to the x-coordinates of the ends of the horizontal line with centre x-coordinate YY(1 0), and length A in either direction from the centre (so a total line length of 2 * A). In other words, this line: X1 YY(1 0) X2 +-----------------+-----------------+ <- A -> <- A -> The resulting line gets clipped to the edges of the screen, if needed. If the calculation doesn't overflow, we return with the C flag clear, otherwise the C flag gets set to indicate failure and the Y-th LSO entry gets set to 0. Arguments: A The half-length of the line YY(1 0) The centre x-coordinate Returns: C flag Clear if the line fits on-screen, set if it doesn't X1, X2 The x-coordinates of the clipped line LSO+Y If the line doesn't fit, LSO+Y is set to 0 Y Y is preserved.CHKON LDA K3 \ Set A = K3 + K CLC ADC K LDA K3+1 \ Set A = K3+1 + 0 + any carry from above, so this ADC #0 \ effectively sets A to the high byte of K3(1 0) + K: \ \ (A ?) = K3(1 0) + K \ \ so A is the high byte of the x-coordinate of the right \ edge of the circle BMI PL21 \ If A is negative then the right edge of the circle is \ to the left of the screen, so jump to PL21 to set the \ C flag and return from the subroutine, as the whole \ circle is off-screen to the left LDA K3 \ Set A = K3 - K SEC SBC K LDA K3+1 \ Set A = K3+1 - 0 - any carry from above, so this SBC #0 \ effectively sets A to the high byte of K3(1 0) - K: \ \ (A ?) = K3(1 0) - K \ \ so A is the high byte of the x-coordinate of the left \ edge of the circle BMI PL31 \ If A is negative then the left edge of the circle is \ to the left of the screen, and we already know the \ right edge is either on-screen or off-screen to the \ right, so skip to PL31 to move on to the y-coordinate \ checks, as at least part of the circle is on-screen in \ terms of the x-axis BNE PL21 \ If A is non-zero, then the left edge of the circle is \ to the right of the screen, so jump to PL21 to set the \ C flag and return from the subroutine, as the whole \ circle is off-screen to the right .PL31 LDA K4 \ Set P+1 = K4 + K CLC ADC K STA P+1 LDA K4+1 \ Set A = K4+1 + 0 + any carry from above, so this ADC #0 \ does the following: \ \ (A P+1) = K4(1 0) + K \ \ so A is the high byte of the y-coordinate of the \ bottom edge of the circle BMI PL21 \ If A is negative then the bottom edge of the circle is \ above the top of the screen, so jump to PL21 to set \ the C flag and return from the subroutine, as the \ whole circle is off-screen to the top STA P+2 \ Store the high byte in P+2, so now we have: \ \ P(2 1) = K4(1 0) + K \ \ i.e. the maximum y-coordinate of the circle on-screen \ (which we return) LDA K4 \ Set X = K4 - K SEC SBC K TAX LDA K4+1 \ Set A = K4+1 - 0 - any carry from above, so this SBC #0 \ does the following: \ \ (A X) = K4(1 0) - K \ \ so A is the high byte of the y-coordinate of the top \ edge of the circle BMI PL44 \ If A is negative then the top edge of the circle is \ above the top of the screen, and we already know the \ bottom edge is either on-screen or below the bottom \ of the screen, so skip to PL44 to clear the C flag and \ return from the subroutine using a tail call, as part \ of the circle definitely appears on-screen BNE PL21 \ If A is non-zero, then the top edge of the circle is \ below the bottom of the screen, so jump to PL21 to set \ the C flag and return from the subroutine, as the \ whole circle is off-screen to the bottom CPX #2*Y-1 \ If we get here then A is zero, which means the top \ edge of the circle is within the screen boundary, so \ now we need to check whether it is in the space view \ (in which case it is on-screen) or the dashboard (in \ which case the top of the circle is hidden by the \ dashboard, so the circle isn't on-screen). We do this \ by checking the low byte of the result in X against \ 2 * #Y - 1, and returning the C flag from this \ comparison. The constant #Y is the y-coordinate of the \ mid-point of the space view, so 2 * #Y - 1 is 191, the \ y-coordinate of the bottom pixel row of the space \ view. So this does the following: \ \ * The C flag is set if coordinate (A X) is below the \ bottom row of the space view, i.e. the top edge of \ the circle is hidden by the dashboard \ \ * The C flag is clear if coordinate (A X) is above \ the bottom row of the space view, i.e. the top \ edge of the circle is on-screen RTS \ Return from the subroutineName: CHKON [View individually] Type: Subroutine Category: Drawing circles Summary: Check whether any part of a circle appears on the extended screen

Arguments: K The circle's radius K3(1 0) Pixel x-coordinate of the centre of the circle K4(1 0) Pixel y-coordinate of the centre of the circle Returns: C flag Clear if any part of the circle appears on-screen, set if none of the circle appears on-screen (A X) Minimum y-coordinate of the circle on-screen (i.e. the y-coordinate of the top edge of the circle) P(2 1) Maximum y-coordinate of the circle on-screen (i.e. the y-coordinate of the bottom edge of the circle).PL21 SEC \ Set the C flag to indicate an overflow RTS \ Return from the subroutineName: PL21 [View individually] Type: Subroutine Category: Drawing planets Summary: Return from a planet/sun-drawing routine with a failure flag

Set the C flag and return from the subroutine. This is used to return from a planet- or sun-drawing routine with the C flag indicating an overflow in the calculation..PLS3 JSR PLS1 \ Call PLS1 to calculate the following: STA P \ \ P = |roofv_x / z| \ K+3 = sign of roofv_x / z \ \ and increment X to point to roofv_y for the next call LDA #222 \ Set Q = 222, the offset to the crater STA Q STX U \ Store the vector index X in U for retrieval after the \ call to MULTU JSR MULTU \ Call MULTU to calculate \ \ (A P) = P * Q \ = 222 * |roofv_x / z| LDX U \ Restore the vector index from U into X LDY K+3 \ If the sign of the result in K+3 is positive, skip to BPL PL12 \ PL12 to return with Y = 0 EOR #&FF \ Otherwise the result should be negative, so negate the CLC \ high byte of the result using two's complement with ADC #1 \ A = ~A + 1 BEQ PL12 \ If A = 0, jump to PL12 to return with (Y A) = 0 LDY #&FF \ Set Y = &FF to be a negative high byte RTS \ Return from the subroutine .PL12 LDY #0 \ Set Y = 0 to be a positive high byte RTS \ Return from the subroutineName: PLS3 [View individually] Type: Subroutine Category: Drawing planets Summary: Calculate (Y A P) = 222 * roofv_x / z

Calculate the following, with X determining the vector to use: (Y A P) = 222 * roofv_x / z though in reality only (Y A) is used. Although the code below supports a range of values of X, in practice the routine is only called with X = 15, and then again after X has been incremented to 17. So the values calculated by PLS1 use roofv_x first, then roofv_y. The comments below refer to roofv_x, for the first call. Arguments: X Determines which of the INWK orientation vectors to divide: * X = 15: divides roofv_x * X = 17: divides roofv_y Returns: X X gets incremented by 2 so it points to the next coordinate in this orientation vector (so consecutive calls to the routine will start with x, then move onto y and then z).PLS4 STA Q \ Set Q = A JSR ARCTAN \ Call ARCTAN to calculate: \ \ A = arctan(P / Q) \ arctan(P / A) LDX INWK+14 \ If nosev_z_hi is negative, skip the following BMI P%+4 \ instruction EOR #%10000000 \ nosev_z_hi is positive, so make the arctan negative LSR A \ Set CNT2 = A / 4 LSR A STA CNT2 RTS \ Return from the subroutineName: PLS4 [View individually] Type: Subroutine Category: Drawing planets Summary: Calculate CNT2 = arctan(P / A) / 4

Calculate the following: CNT2 = arctan(P / A) / 4 giving the result the opposite sign to nosev_z_hi. This is called with the following arguments when calculating the equator and meridian for planets: * A = roofv_z_hi, P = -nosev_z_hi * A = sidev_z_hi, P = -nosev_z_hi So it calculates the angle between the planet's orientation vectors, in the z-axis..PLS5 JSR PLS1 \ Call PLS1 to calculate the following: STA K2+2 \ STY XX16+2 \ K+2 = |roofv_x / z| \ XX16+2 = sign of roofv_x / z \ \ i.e. (XX16+2 K2+2) = roofv_x / z \ \ and increment X to point to roofv_y for the next call JSR PLS1 \ Call PLS1 to calculate the following: STA K2+3 \ STY XX16+3 \ K+3 = |roofv_y / z| \ XX16+3 = sign of roofv_y / z \ \ i.e. (XX16+3 K2+3) = roofv_y / z \ \ and increment X to point to roofv_z for the next call RTS \ Return from the subroutineName: PLS5 [View individually] Type: Subroutine Category: Drawing planets Summary: Calculate roofv_x / z and roofv_y / z

Calculate the following divisions of a specified value from one of the orientation vectors (in this example, roofv): (XX16+2 K2+2) = roofv_x / z (XX16+3 K2+3) = roofv_y / z Arguments: X Determines which of the INWK orientation vectors to divide: * X = 15: divides roofv_x and roofv_y * X = 21: divides sidev_x and sidev_y INWK The planet's ship data block.PLS6 JSR DVID3B2 \ Call DVID3B2 to calculate: \ \ K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo) LDA K+3 \ Set A = |K+3| OR K+2 AND #%01111111 ORA K+2 BNE PL21 \ If A is non-zero then the two high bytes of K(3 2 1 0) \ are non-zero, so jump to PL21 to set the C flag and \ return from the subroutine \ We can now just consider K(1 0), as we know the top \ two bytes of K(3 2 1 0) are both 0 LDX K+1 \ Set X = K+1, so now (X K) contains the result in \ K(1 0), which is the format we want to return the \ result in CPX #4 \ If the high byte of K(1 0) >= 4 then the result is BCS PL6 \ >= 1024, so return from the subroutine with the C flag \ set to indicate an overflow (as PL6 contains an RTS) LDA K+3 \ Fetch the sign of the result from K+3 (which we know \ has zeroes in bits 0-6, so this just fetches the sign) \CLC \ This instruction is commented out in the original \ source. It would have no effect as we know the C flag \ is already clear, as we skipped past the BCS above BPL PL6 \ If the sign bit is clear and the result is positive, \ then the result is already correct, so return from \ the subroutine with the C flag clear to indicate \ success (as PL6 contains an RTS) LDA K \ Otherwise we need to negate the result, which we do EOR #%11111111 \ using two's complement, starting with the low byte: ADC #1 \ STA K \ K = ~K + 1 TXA \ And then the high byte: EOR #%11111111 \ ADC #0 \ X = ~X TAX .PL44 CLC \ Clear the C flag to indicate success .PL6 RTS \ Return from the subroutineName: PLS6 [View individually] Type: Subroutine Category: Drawing planets Summary: Calculate (X K) = (A P) / (z_sign z_hi z_lo)

Calculate the following: (X K) = (A P) / (z_sign z_hi z_lo) returning an overflow in the C flag if the result is >= 1024. Arguments: INWK The planet or sun's ship data block Returns: C flag Set if the result >= 1024, clear otherwise Other entry points: PL44 Clear the C flag and return from the subroutine.TT17 JSR DOKEY \ Scan the keyboard for flight controls and pause keys, \ (or the equivalent on joystick) and update the key \ logger, setting KL to the key pressed LDA JSTK \ If the joystick was not used, jump down to TJ1, BEQ TJ1 \ otherwise we move the cursor with the joystick LDA JSTX \ Fetch the joystick roll, ranging from 1 to 255 with \ 128 as the centre point EOR #&FF \ Flip the sign so A = -JSTX, because the joystick roll \ works in the opposite way to moving a cursor on-screen \ in terms of left and right JSR TJS1 \ Call TJS1 just below to set Y to a value between -2 \ and +2 depending on the joystick roll value (moving \ the stick sideways) TYA \ Copy Y to A TAX \ Copy A to X LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point, and fall through into TJS1 to \ joystick pitch value (moving the stick up and down) .TJS1 TAY \ Store A in Y LDA #0 \ Set the result, A = 0 CPY #&10 \ If Y >= &10 set the C flag, so A = A - 1 SBC #0 \CPY #&20 \ These instructions are commented out in the original \SBC #0 \ source, but they would make the joystick move the \ cursor faster by increasing the range of Y by -1 to +1 CPY #64 \ If Y >= 64 set the C flag, so A = A - 1 SBC #0 CPY #192 \ If Y >= 192 set the C flag, so A = A + 1 ADC #0 CPY #224 \ If Y >= 224 set the C flag, so A = A + 1 ADC #0 \CPY #&F0 \ These instructions are commented out in the original \ADC #0 \ source, but they would make the joystick move the \ cursor faster by increasing the range of Y by -1 to +1 TAY \ Copy the value of A into Y LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine .TJ1 LDA KL \ Set A to the value of KL (the key pressed) LDX #0 \ Set the results, X = Y = 0 LDY #0 CMP #&19 \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&79 \ If right arrow was pressed, set X = X + 1 BNE P%+3 INX CMP #&39 \ If up arrow was pressed, set Y = Y + 1 BNE P%+3 INY CMP #&29 \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY RTS \ Return from the subroutineName: TT17 [View individually] Type: Subroutine Category: Keyboard Summary: Scan the keyboard for cursor key or joystick movement

Scan the keyboard and joystick for cursor key or stick movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * For joystick, X and Y are integers between -2 and +2 depending on how far the stick has moved * For keyboard, X and Y are integers between -1 and +1 depending on which keys are pressed Returns: A The key pressed, if the arrow keys were used X Change in the x-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above) Y Change in the y-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above).ping LDX #1 \ We want to copy the X- and Y-coordinates of the \ current system in (QQ0, QQ1) to the selected system's \ coordinates in (QQ9, QQ10), so set up a counter to \ copy two bytes .pl1 LDA QQ0,X \ Load byte X from the current system in QQ0/QQ1 STA QQ9,X \ Store byte X in the selected system in QQ9/QQ10 DEX \ Decrement the loop counter BPL pl1 \ Loop back for the next byte to copy RTS \ Return from the subroutineName: ping [View individually] Type: Subroutine Category: Universe Summary: Set the selected system to the current systemPRINT "ELITE E" PRINT "Assembled at ", ~CODE_E% PRINT "Ends at ", ~P% PRINT "Code size is ", ~(P% - CODE_E%) PRINT "Execute at ", ~LOAD% PRINT "Reload at ", ~LOAD_E% PRINT "S.ELTE ", ~CODE_E%, " ", ~P%, " ", ~LOAD%, " ", ~LOAD_E% SAVE "output/ELTE.bin", CODE_E%, P%, LOAD%Save output/ELTE.bin

[X]

Subroutine ADD (category: Maths (Arithmetic))

Calculate (A X) = (A P) + (S R)

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Subroutine ARCTAN (category: Maths (Geometry))

Calculate A = arctan(P / Q)

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Subroutine BLINE (category: Drawing circles)

Draw a circle segment and add it to the ball line heap

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Subroutine BPRNT (category: Text)

Print a 32-bit number, left-padded to a specific number of digits, with an optional decimal point

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Entry point BULB-2 in subroutine SPBLB (category: Dashboard)

Set the Y screen address

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Subroutine CHKON (category: Drawing circles)

Check whether any part of a circle appears on the extended screen

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Subroutine CIRCLE (category: Drawing circles)

Draw a circle for the planet

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Subroutine CPIX2 (category: Drawing pixels)

Draw a single-height dot on the dashboard

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Variable CTWOS (category: Drawing pixels)

Ready-made single-pixel character row bytes for mode 5

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Subroutine DEATH (category: Start and end)

Display the death screen

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Subroutine DOKEY (category: Keyboard)

Scan for the seven primary flight controls

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Subroutine DORND (category: Utility routines)

Generate random numbers

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Entry point DORND2 in subroutine DORND (category: Utility routines)

Restricts the value of RAND+2 so that bit 0 is always 0

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Subroutine DOT (category: Dashboard)

Draw a dot on the compass

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Subroutine DVID3B2 (category: Maths (Arithmetic))

Calculate K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo)

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Subroutine DVID4 (category: Maths (Arithmetic))

Calculate (P R) = 256 * A / Q

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Variable ECBT (category: Dashboard)

The character definition for the E.C.M. indicator

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Subroutine EDGES (category: Drawing lines)

Draw a horizontal line given a centre and a half-width

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Subroutine EXNO3 (category: Sound)

Make an explosion sound

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Subroutine FMLTU (category: Maths (Arithmetic))

Calculate A = A * Q / 256

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Subroutine FMLTU2 (category: Maths (Arithmetic))

Calculate A = K * sin(A)

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Subroutine GINF (category: Universe)

Fetch the address of a ship's data block into INF

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Subroutine HLOIN (category: Drawing lines)

Draw a horizontal line from (X1, Y1) to (X2, Y1)

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Subroutine HLOIN2 (category: Drawing lines)

Remove a line from the sun line heap and draw it on-screen

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Workspace K% (category: Workspaces)

Ship data blocks and ship line heaps

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Subroutine LL5 (category: Maths (Arithmetic))

Calculate Q = SQRT(R Q)

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Subroutine LOIN (Part 1 of 7) (category: Drawing lines)

Draw a line: Calculate the line gradient in the form of deltas

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Subroutine MSBAR (category: Dashboard)

Draw a specific indicator in the dashboard's missile bar

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Subroutine MULTU (category: Maths (Arithmetic))

Calculate (A P) = P * Q

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Variable NA% (category: Save and load)

The data block for the last saved commander

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Configuration variable: NI% = 36

The number of bytes in each ship's data block (as stored in INWK and K%)

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Subroutine NOISE (category: Sound)

Make the sound whose number is in A

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Configuration variable: NOSH = 12

The maximum number of ships in our local bubble of universe (counting from 0, so there are actually 13 ship slots)

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Subroutine NWSHP (category: Universe)

Add a new ship to our local bubble of universe

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Subroutine NwS1 (category: Universe)

Flip the sign and double an INWK byte

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Subroutine OUCH (category: Flight)

Potentially lose cargo or equipment following damage

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Subroutine PIXEL (category: Drawing pixels)

Draw a 1-pixel dot, 2-pixel dash or 4-pixel square

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Subroutine PIXEL2 (category: Drawing pixels)

Draw a stardust particle relative to the screen centre

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Subroutine PL2 (category: Drawing planets)

Remove the planet or sun from the screen

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Entry point PL2-1 in subroutine PL2 (category: Drawing planets)

Contains an RTS

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Label PL20 in subroutine PL9 (Part 1 of 3)

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Subroutine PL21 (category: Drawing planets)

Return from a planet/sun-drawing routine with a failure flag

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Label PL25 in subroutine PL9 (Part 1 of 3)

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Label PL26 in subroutine PL9 (Part 3 of 3)

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Entry point PL44 in subroutine PLS6 (category: Drawing planets)

Clear the C flag and return from the subroutine

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Subroutine PL9 (Part 1 of 3) (category: Drawing planets)

Draw the planet, with either an equator and meridian, or a crater

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Label PLF10 in subroutine SUN (Part 3 of 4)

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Label PLF11 in subroutine SUN (Part 3 of 4)

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Label PLF13 in subroutine SUN (Part 2 of 4)

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Label PLF16 in subroutine SUN (Part 3 of 4)

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Label PLF17 in subroutine SUN (Part 1 of 4)

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Label PLF2 in subroutine SUN (Part 1 of 4)

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Label PLF23 in subroutine SUN (Part 3 of 4)

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Label PLF3 in subroutine SUN (Part 1 of 4)

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[X]

Label PLF4 in subroutine SUN (Part 1 of 4)

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Label PLF44 in subroutine SUN (Part 3 of 4)

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Label PLF5 in subroutine SUN (Part 1 of 4)

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Label PLF6 in subroutine SUN (Part 3 of 4)

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Label PLF8 in subroutine SUN (Part 4 of 4)

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Label PLF9 in subroutine SUN (Part 4 of 4)

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Label PLFL in subroutine SUN (Part 3 of 4)

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Label PLFL2 in subroutine SUN (Part 2 of 4)

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Label PLFL3 in subroutine SUN (Part 4 of 4)

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Label PLFLS in subroutine SUN (Part 3 of 4)

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Subroutine PLS1 (category: Drawing planets)

Calculate (Y A) = nosev_x / z

[X]

Subroutine PLS2 (category: Drawing planets)

Draw a half-circle

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Subroutine PLS22 (category: Drawing planets)

Draw a circle or half-circle

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Subroutine PLS3 (category: Drawing planets)

Calculate (Y A P) = 222 * roofv_x / z

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Subroutine PLS4 (category: Drawing planets)

Calculate CNT2 = arctan(P / A) / 4

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Subroutine PLS5 (category: Drawing planets)

Calculate roofv_x / z and roofv_y / z

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Subroutine PLS6 (category: Drawing planets)

Calculate (X K) = (A P) / (z_sign z_hi z_lo)

[X]

Subroutine PROJ (category: Drawing ships)

Project the current ship onto the screen

[X]

Variable QQ16 (category: Text)

The two-letter token lookup table

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Variable QQ18 (category: Text)

The recursive token table for tokens 0-148

[X]

Entry point RREN in subroutine TT26 (category: Text)

Prints the character definition pointed to by P(2 1) at the screen address pointed to by (A SC). Used by the BULB routine

[X]

Entry point RTS2 in subroutine SUN (Part 1 of 4) (category: Drawing suns)

Contains an RTS

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Subroutine SCAN (category: Dashboard)

Display the current ship on the scanner

[X]

Subroutine SHD (category: Flight)

Charge a shield and drain some energy from the energy banks

[X]

[X]

Configuration variable: SHEILA = &FE00

Memory-mapped space for accessing internal hardware, such as the video ULA, 6845 CRTC and 6522 VIAs

[X]

Variable SNE (category: Maths (Geometry))

Sine/cosine table

[X]

Subroutine SOS1 (category: Universe)

Update the missile indicators, set up the planet data block

[X]

Subroutine SP1 (category: Dashboard)

Draw the space station on the compass

[X]

Subroutine SP2 (category: Dashboard)

Draw a dot on the compass, given the planet/station vector

[X]

Subroutine SPBLB (category: Dashboard)

Light up the space station indicator ("S") on the dashboard

[X]

Variable SPBT (category: Dashboard)

The character definition for the space station indicator

[X]

Subroutine SPS1 (category: Maths (Geometry))

Calculate the vector to the planet and store it in XX15

[X]

Subroutine SPS2 (category: Maths (Arithmetic))

Calculate (Y X) = A / 10

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Subroutine SPS4 (category: Maths (Geometry))

Calculate the vector to the space station

[X]

Subroutine SQUA2 (category: Maths (Arithmetic))

Calculate (A P) = A * A

[X]

Configuration variable: SST = 8

Ship type for the space station

[X]

Subroutine SUN (Part 1 of 4) (category: Drawing suns)

Draw the sun: Set up all the variables needed

[X]

Subroutine TAS2 (category: Maths (Geometry))

Normalise the three-coordinate vector in K3

[X]

Subroutine TT26 (category: Text)

Print a character at the text cursor (WRCHV points here)

[X]

Subroutine TT27 (category: Text)

Print a text token

[X]

Subroutine TT41 (category: Text)

Print a letter according to Sentence Case

[X]

Subroutine TT42 (category: Text)

Print a letter in lower case

[X]

Subroutine TT43 (category: Text)

Print a two-letter token or recursive token 0-95

[X]

Entry point TT44 in subroutine TT42 (category: Text)

Jumps to TT26 to print the character in A (used to enable us to use a branch instruction to jump to TT26)

[X]

Subroutine TT45 (category: Text)

Print a letter in lower case

[X]

Subroutine TT46 (category: Text)

Print a character and switch to capitals

[X]

Entry point TT48 in subroutine ex (category: Text)

Contains an RTS

[X]

Subroutine TT54 (category: Universe)

Twist the selected system's seeds

[X]

Subroutine TT67 (category: Text)

Print a newline

[X]

Subroutine TT68 (category: Text)

Print a text token followed by a colon

[X]

Subroutine TT73 (category: Text)

Print a colon

[X]

Subroutine TT74 (category: Text)

Print a character

[X]

Variable UNIV (category: Universe)

Table of pointers to the local universe's ship data blocks

[X]

Subroutine WPLS (category: Drawing suns)

Remove the sun from the screen

[X]

Entry point WPLS-1 in subroutine WPLS2 (category: Drawing planets)

Contains an RTS

[X]

Subroutine WPLS2 (category: Drawing planets)

Remove the planet from the screen

[X]

Subroutine WPSHPS (category: Dashboard)

Clear the scanner, reset the ball line and sun line heaps

[X]

Configuration variable: X = 128

The centre x-coordinate of the 256 x 192 space view

[X]

Temporary storage, used to store the address of a ship blueprint. For example, it is used when we add a new ship to the local bubble in routine NWSHP, and it contains the address of the current ship's blueprint as we loop through all the nearby ships in the main flight loop

[X]

Variable XX21 (category: Drawing ships)

Ship blueprints lookup table

[X]

Workspace XX3 (category: Workspaces)

Temporary storage space for complex calculations

[X]

Configuration variable: Y = 96

The centre y-coordinate of the 256 x 192 space view

[X]

Subroutine ZINF (category: Utility routines)

Reset the INWK workspace

[X]

Subroutine cmn (category: Text)

Print the commander's name

[X]

Entry point cmn-1 in subroutine cpl (category: Text)

Contains an RTS

[X]

Subroutine cpl (category: Text)

Print the selected system name

[X]

Subroutine crlf (category: Text)

Tab to column 21 and print a colon

[X]

Subroutine csh (category: Text)

Print the current amount of cash

[X]

Subroutine ex (category: Text)

Print a recursive token

[X]

Subroutine fwl (category: Text)

Print fuel and cash levels

[X]

Subroutine msblob (category: Dashboard)

Display the dashboard's missile indicators in green

[X]

Subroutine plf (category: Text)

Print a text token followed by a newline

[X]

Subroutine pr2 (category: Text)

Print an 8-bit number, left-padded to 3 digits, and optional point

[X]

Subroutine qw (category: Text)

Print a recursive token in the range 128-145

[X]

Subroutine tal (category: Text)

Print the current galaxy numbe

[X]

Subroutine ypl (category: Text)

Print the current system name

[X]

Entry point ypl-1 in subroutine cmn (category: Text)

Contains an RTS