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Elite on the BBC Micro

Elite E source [Electron version]

ELITE E FILE Produces the binary file ELTE.bin that gets loaded by elite-bcfs.asm.
CODE_E% = P% LOAD_E% = LOAD% + P% - CODE%
Name: Authors' names [Show more] Type: Variable Category: Copy protection Summary: The authors' names and a copyright notice, buried in the code
Context: See this variable on its own page Variations: See code variations for this variable in the different versions References: No direct references to this variable in this source file

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)BellBraben'84
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 'B' EOR 164 EQUB 'r' EOR 164 EQUB 'a' EOR 164 EQUB 'b' EOR 164 EQUB 'e' EOR 164 EQUB 'n' EOR 164 EQUB ''' EOR 164 EQUB '8' EOR 164 EQUB '4' EOR 164
Name: cpl [Show more] Type: Subroutine Category: Text Summary: Print the selected system name Deep dive: Generating system names Galaxy and system seeds
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT102 calls cpl * TT23 calls cpl * TT27 calls cpl * hyp calls cpl * ypl calls cpl

Print control code 3 (the selected system name, i.e. the one in the crosshairs in the Short-range Chart).
.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 s0_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 s2_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 \ two 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 \ subroutine
Name: cmn [Show more] Type: Subroutine Category: Text Summary: Print the commander's name
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * TT27 calls cmn

Print control code 4 (the commander's name). 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 subroutine
Name: ypl [Show more] Type: Subroutine Category: Text Summary: Print the current system name
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * TT27 calls ypl * cmn calls entry point ypl-1

Print control code 2 (the current system name). Other entry points: ypl-1 Contains an RTS
.ypl 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 \ subroutine
Name: tal [Show more] Type: Subroutine Category: Text Summary: Print the current galaxy number
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls tal

Print control code 1 (the current galaxy number, right-aligned to width 3).
.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 return from the subroutine using a tail call
Name: fwl [Show more] Type: Subroutine Category: Text Summary: Print fuel and cash levels
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT213 calls fwl * TT27 calls fwl

Print control code 5 ("FUEL: ", fuel level, " LIGHT YEARS", newline, "CASH:", control code 0).
.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 LDA #119 \ Print recursive token 119 ("CASH:" then control code BNE TT27 \ 0, which prints cash levels, then " CR" and newline)
Name: csh [Show more] Type: Subroutine Category: Text Summary: Print the current amount of cash
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls csh

Print control code 0 (the current amount of cash, right-aligned to width 9, followed by " CR" and a newline).
.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 amount 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 cash amount 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 plf
Name: plf [Show more] Type: Subroutine Category: Text Summary: Print a text token followed by a newline
Context: See this subroutine on its own page References: This subroutine is called as follows: * STATUS calls plf * TITLE calls plf * fwl calls plf * plf2 calls plf

Arguments: A The text token to be printed
.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 call
Name: TT68 [Show more] Type: Subroutine Category: Text Summary: Print a text token followed by a colon
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT146 calls TT68 * TT208 calls TT68 * TT25 calls TT68 * fwl calls TT68

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 colon
Name: TT73 [Show more] Type: Subroutine Category: Text Summary: Print a colon
Context: See this subroutine on its own page References: This subroutine is called as follows: * crlf calls TT73
.TT73 LDA #':' \ Set A to ASCII ":" and fall through into TT27 to \ actually print the colon
Name: TT27 [Show more] Type: Subroutine Category: Text Summary: Print a text token Deep dive: Printing text tokens
Context: See this subroutine on its own page References: This subroutine is called as follows: * EQSHP calls TT27 * GTNME calls TT27 * NLIN3 calls TT27 * TITLE calls TT27 * TT151 calls TT27 * TT162 calls TT27 * TT208 calls TT27 * TT210 calls TT27 * TT213 calls TT27 * TT214 calls TT27 * TT219 calls TT27 * TT22 calls TT27 * TT25 calls TT27 * TT43 calls TT27 * TT60 calls TT27 * TT67 calls TT27 * TT68 calls TT27 * TT70 calls TT27 * TTX66 calls TT27 * cpl calls TT27 * ex calls TT27 * fwl calls TT27 * hy6 calls TT27 * hyp calls TT27 * mes9 calls TT27 * plf calls TT27 * prq calls TT27 * qv calls TT27 * spc calls TT27

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
.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 galaxy BEQ tal \ 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 three 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 two 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 two 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: TT42 [Show more] Type: Subroutine Category: Text Summary: Print a letter in lower case
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT45 calls TT42 * TT41 calls entry point TT44

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)
.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 A
Name: TT41 [Show more] Type: Subroutine Category: Text Summary: Print a letter according to Sentence Case
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls TT41

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
.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: qw [Show more] Type: Subroutine Category: Text Summary: Print a recursive token in the range 128-145
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls qw

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
.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 it
Name: crlf [Show more] Type: Subroutine Category: Text Summary: Tab to column 21 and print a colon
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * TT27 calls crlf

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.
.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: TT45 [Show more] Type: Subroutine Category: Text Summary: Print a letter in lower case
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT41 calls TT45

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
.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 letter
Name: TT46 [Show more] Type: Subroutine Category: Text Summary: Print a character and switch to capitals
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls TT46

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
.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 character
Name: TT74 [Show more] Type: Subroutine Category: Text Summary: Print a character
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls TT74 * TT41 calls TT74

Arguments: A The character to be printed
.TT74 JMP TT26 \ Print the character in A
Name: TT43 [Show more] Type: Subroutine Category: Text Summary: Print a two-letter token or recursive token 0-95
Context: See this subroutine on its own page References: This subroutine is called as follows: * TT27 calls TT43

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)
.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 it
Name: ex [Show more] Type: Subroutine Category: Text Summary: Print a recursive token
Context: See this subroutine on its own page References: This subroutine is called as follows: * DEATH calls ex * TITLE calls ex * TT27 calls ex * qw calls ex * DOEXP calls entry point TT48 * TT43 calls entry point TT48 * TT45 calls entry point TT48

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
.ex TAX \ Copy the token number into X LDA #LO(QQ18) \ Set V(1 0) 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 subroutine
Name: DOEXP [Show more] Type: Subroutine Category: Drawing ships Summary: Draw an exploding ship Deep dive: Drawing explosion clouds Generating random numbers
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LL9 (Part 1 of 12) calls DOEXP * LL9 (Part 9 of 12) calls DOEXP
.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 16 so that is has a maximum value of 7 LSR A 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, making sure the C flag STA ZZ \ doesn't affect the outcome 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, making sure the C flag \ doesn't affect the outcome 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, making sure the C flag \ doesn't affect the outcome 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 subroutine
Name: SOS1 [Show more] Type: Subroutine Category: Universe Summary: Update the missile indicators, set up the planet data block
Context: See this subroutine on its own page References: This subroutine is called as follows: * SOLAR calls SOS1 * TT110 calls SOS1

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).
.SOS1 JSR msblob \ Reset the dashboard's missile indicators so none of \ them are targeted 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: SOLAR [Show more] Type: Subroutine Category: Universe Summary: Set up various aspects of arriving in a new system
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * TT18 calls SOLAR

Halve our legal status, update the missile indicators, and set up the data block and slot for the planet.
.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 s0_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, at the same time shifting bit 0 of the result \ of the addition into the C flag 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 s0_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 #129 \ Set A = 129, the ship type for the placeholder, so \ there isn't a space station, but there is a non-zero \ ship type to indicate this JSR NWSHP \ Call NWSHP to set up the new data block and add it \ to FRIN, where it will get put in the second slot as \ we just cleared out the second slot, and the first \ slot is already taken by the planet
Name: NWSTARS [Show more] Type: Subroutine Category: Stardust Summary: Initialise the stardust field
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LOOK1 calls NWSTARS

This routine is called when the space view is initialised in routine LOOK1.
.NWSTARS LDA QQ11 \ If this is not a space view, jump to WPSHPS to skip BNE WPSHPS \ the initialisation of the SX, SY and SZ tables
Name: nWq [Show more] Type: Subroutine Category: Stardust Summary: Create a random cloud of stardust
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DEATH calls nWq

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.
.nWq LDY #NOST \ Set Y to the 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 block
Name: WPSHPS [Show more] Type: Subroutine Category: Dashboard Summary: Clear the scanner and reset the ball line heap
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LOOK1 calls WPSHPS * NWSTARS calls WPSHPS * RES2 calls WPSHPS

Remove all ships from the scanner and reset the ball line heap at LSX2 and LSY2.
.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, so jump down to WS1 to skip \ this slot, as the planet 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 block
Name: SHD [Show more] Type: Subroutine Category: Flight Summary: Charge a shield and drain some energy from the energy banks
Context: See this subroutine on its own page References: This subroutine is called as follows: * Main flight loop (Part 13 of 16) calls SHD

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
DEX \ 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 charging
Name: DENGY [Show more] Type: Subroutine Category: Flight Summary: Drain some energy from the energy banks
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LASLI calls DENGY * Main flight loop (Part 16 of 16) calls DENGY

Returns: Z flag Set if we have no energy left, clear otherwise
.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 subroutine
Name: COMPAS [Show more] Type: Subroutine Category: Dashboard Summary: Update the compass
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DIALS (Part 4 of 4) calls COMPAS
.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 using a tail call
Name: SPS2 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (Y X) = A / 10
Context: See this subroutine on its own page References: This subroutine is called as follows: * SP2 calls SPS2

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
.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 subroutine
Name: SPS4 [Show more] Type: Subroutine Category: Maths (Geometry) Summary: Calculate the vector to the space station
Context: See this subroutine on its own page References: This subroutine is called as follows: * Main flight loop (Part 9 of 16) calls SPS4 * SP1 calls SPS4

Calculate the vector between our ship and the space station and store it in XX15.
.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 call
Name: SP1 [Show more] Type: Subroutine Category: Dashboard Summary: Draw the space station on the compass
Context: See this subroutine on its own page References: This subroutine is called as follows: * COMPAS calls SP1
.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 compass
Name: SP2 [Show more] Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass, given the planet/station vector
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * COMPAS calls SP2

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
.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 = 193 + X, as 184 is the pixel x-coordinate ADC #193 \ of the leftmost edge of the compass, and X can be -9, STA COMX \ which would be 193 - 9 = 184. This also means that the \ highest value for COMX is 193 + 9 = 202, and given \ that the compass dot is two pixels wide, this means \ the compass dot can overlap the left edge of the \ compass, but not the right 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 &F0, the value we pass to DOT for drawing a \ two-pixel high dot, 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 a \ single-height dash, so set A to &FF for the call to \ DOT below STA COMC \ Store the compass shape in COMC \ Fall through into DOT to draw the dot on the compass
Name: DOT [Show more] Type: Subroutine Category: Dashboard Summary: Draw a dash on the compass
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * COMPAS calls DOT

Arguments: COMX The screen pixel x-coordinate of the dash COMY The screen pixel y-coordinate of the dash COMC The thickness of the dash: * &F0 = a double-height dash in white, for when the object in the compass is in front of us * &FF = a single-height dash in white, for when the object in the compass is behind us
.DOT LDA COMY \ Set Y1 = COMY, the y-coordinate of the dash STA Y1 LDA COMX \ Set X1 = COMX, the x-coordinate of the dash STA X1 LDA COMC \ Set A = COMC, the thickness of the dash CMP #&F0 \ If COMC is &F0 then the planet/station is in front of BNE CPIX2 \ us and we want to draw a double-height dash, so if it \ isn't &F0 jump to CPIX2 to draw a single-height dash \ Otherwise fall through into CPIX4 to draw a double- \ height dash
Name: CPIX4 [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a double-height dot on the dashboard
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * SCAN calls CPIX4 * TT23 calls CPIX4

Draw a double-height mode 4 dot (2 pixels high, 4 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
.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 dot
Name: CPIX2 [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a single-height dash on the dashboard Deep dive: Drawing pixels in the Electron version
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * CPIX4 calls CPIX2 * DOT calls CPIX2

Draw a single-height mode 4 dash (1 pixel high, 4 pixels wide). Arguments: X1 The screen pixel x-coordinate of the dash Y1 The screen pixel y-coordinate of the dash
.CPIX2 LDY #128 \ Set SC = 128 for use in the calculation below STY SC LDA Y1 \ Fetch the y-coordinate into A \ We now calculate the address of the character block \ containing the pixel (x, y) and put it in SC(1 0), as \ follows: \ \ SC = &5800 + (y div 8 * 256) + (y div 8 * 64) + 32 \ \ See the deep dive on "Drawing pixels in the Electron \ version" for details LSR A \ Set A = A >> 3 LSR A \ = y div 8 LSR A \ = character row number \ Also, as SC = 128, we have: \ \ (A SC) = (A 128) \ = (A * 256) + 128 \ = 4 * ((A * 64) + 32) \ = 4 * ((char row * 64) + 32) STA SC+1 \ Set SC+1 = A, so (SC+1 0) = A * 256 \ = char row * 256 LSR A \ Set (A SC) = (A SC) / 4 ROR SC \ = (4 * ((char row * 64) + 32)) / 4 LSR A \ = char row * 64 + 32 ROR SC ADC SC+1 \ Set SC(1 0) = (A SC) + (SC+1 0) + &5800 ADC #&58 \ = (char row * 64 + 32) STA SC+1 \ + char row * 256 \ + &5800 \ \ which is what we want, so SC(1 0) contains the address \ of the first visible pixel on the character row \ containing the point (x, y) LDA X1 \ Each character block contains 8 pixel rows, so to get AND #%11111000 \ 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 ADC SC \ And add the result to SC(1 0) to get the character STA SC \ block on the row we want BCC P%+4 \ If the addition of the low bytes overflowed, increment INC SC+1 \ the high byte \ So SC(1 0) now contains the address of the first pixel \ in the character block containing the (x, y), taking \ the screen borders into consideration LDA Y1 \ 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 \ Set X to just bits 0-2 of the x-coordinate, which will AND #%00000111 \ be the pixel number within the character row we need TAX \ to draw LDA TWOS,X \ Fetch a mode 4 1-pixel byte with the pixel position \ at X 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 JSR P%+3 \ Run the following code twice, incrementing X each \ time, so we draw a two-pixel dash INX \ Increment X to get the next pixel along LDA TWOS,X \ Fetch a mode 4 1-pixel byte with the pixel position \ at X BPL CP1 \ The CTWOS table is followed by the TWOS2 table, whose \ first entry is %11000000, so if we have not just \ 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 CLC \ position of four in this character block, so we add ADC #8 \ 8 to the screen address to move onto the next block STA SC \ along (as there are 8 bytes in a character block) BCC P%+4 \ If the addition we just did overflowed, then increment INC SC+1 \ the high byte of SC(1 0), as this means we just moved \ into the right half of the screen row LDA TWOS,X \ Refetch the mode 4 1-pixel byte from before, as we \ just overwrote A .CP1 EOR (SC),Y \ Draw the dash's right pixel according to the mask in STA (SC),Y \ A, using EOR logic, just as above RTS \ Return from the subroutine
Name: OOPS [Show more] Type: Subroutine Category: Flight Summary: Take some damage
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main flight loop (Part 10 of 16) calls OOPS * TACTICS (Part 1 of 7) calls OOPS * TACTICS (Part 6 of 7) calls OOPS

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
.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 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 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 call
Name: SPS3 [Show more] Type: Subroutine Category: Maths (Geometry) Summary: Copy a space coordinate from the K% block into K3
Context: See this subroutine on its own page References: This subroutine is called as follows: * SPS1 calls SPS3

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 copying the planet's x-coordinate into K3(2 1 0). 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)
.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 subroutine
Name: GINF [Show more] Type: Subroutine Category: Universe Summary: Fetch the address of a ship's data block into INF
Context: See this subroutine on its own page References: This subroutine is called as follows: * FRMIS calls GINF * Main flight loop (Part 4 of 16) calls GINF * NWSHP calls GINF * WPSHPS calls GINF

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
.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 subroutine
Name: NWSPS [Show more] Type: Subroutine Category: Universe Summary: Add a new space station to our local bubble of universe
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main flight loop (Part 14 of 16) calls NWSPS * TT110 calls NWSPS
.NWSPS JSR SPBLB \ Light up the space station bulb on the dashboard LDX #%00000001 \ Set the AI flag in byte #32 to %00000001 (friendly, no STX INWK+32 \ AI, has an E.C.M.) DEX \ Set pitch counter to 0 (no pitch, roll only) STX INWK+30 STX FRIN+1 \ Set the space station slot at FRIN+1 to 0, to indicate \ we should show the space station 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 \ universe
Name: NWSHP [Show more] Type: Subroutine Category: Universe Summary: Add a new ship to our local bubble of universe
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * FRS1 calls NWSHP * KS4 calls NWSHP * Main game loop (Part 1 of 6) calls NWSHP * Main game loop (Part 2 of 6) calls NWSHP * Main game loop (Part 3 of 6) calls NWSHP * Main game loop (Part 4 of 6) calls NWSHP * SFS1 calls NWSHP * SOLAR calls NWSHP * SOS1 calls NWSHP * TITLE calls NWSHP

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) INF Points to the new ship's data block in K%
.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 (note that this means \ only slots from 0 to #NOSH - 1 are populated by this \ routine, but there is one more slot reserved in FRIN, \ which is used to identify the end of the slot list \ when shuffling the slots down in the KILLSHP routine) .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 the planet, so \ jump down to NW2, as the next section sets up a ship \ data block, which doesn't apply to planets, 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, so XX0(1 0) now \ contains the address of this ship's blueprint 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 \ 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 SBC INF \ arithmetic, and put the result in (A Y), so the high TAY \ byte is in A and the low byte in Y. The subtraction LDA INWK+34 \ works because the previous subtraction will never SBC INF+1 \ underflow, so we know the C flag is set 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 returns \ from the subroutine (with the C flag already cleared) 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 returns \ from the subroutine (with the C flag already cleared) .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 (i.e. the planet), 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 subroutine
Name: NwS1 [Show more] Type: Subroutine Category: Universe Summary: Flip the sign and double an INWK byte
Context: See this subroutine on its own page References: This subroutine is called as follows: * NWSPS calls NwS1

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
.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 subroutine
Name: ABORT [Show more] Type: Subroutine Category: Dashboard Summary: Disarm missiles and update the dashboard indicators
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * FRMIS calls ABORT * KILLSHP calls entry point ABORT-2 * Main flight loop (Part 3 of 16) calls entry point ABORT-2

Arguments: Y The new status of the leftmost missile indicator Other entry points: ABORT-2 Set the indicator to disarmed (white square)
LDY #&09 \ Set Y = &09 so we set the missile to a white square \ (disarmed) .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 missile
Name: ABORT2 [Show more] Type: Subroutine Category: Dashboard Summary: Set/unset the lock target for a missile and update the dashboard
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main flight loop (Part 11 of 16) calls ABORT2

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 shape of the missile indicator: * &04 = black (no missile) * &11 = black "T" in white square (armed and locked) * &0D = black box in white square (armed) * &09 = white square (disarmed)
.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 subroutine
Name: ECBLB2 [Show more] Type: Subroutine Category: Dashboard Summary: Start up the E.C.M. (indicator, start countdown and make sound)
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main flight loop (Part 3 of 16) calls ECBLB2 * TACTICS (Part 1 of 7) calls ECBLB2

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.
.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. bulb
Name: ECBLB [Show more] Type: Subroutine Category: Dashboard Summary: Light up the E.C.M. indicator bulb ("E") on the dashboard
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * ECMOF calls ECBLB
.ECBLB LDA #&98 \ Set A to the low byte of the screen address of the \ E.C.M. bulb (which is at &7C98) LDX #LO(ECBT) \ Set X to the low byte of the address of the character \ definition in ECBT LDY #&7C \ Set Y to the high byte of the screen address of the \ E.C.M. bulb (which is at &7C98) BNE BULB \ Jump down to BULB (this BNE is effectively a JMP as \ A will never be zero)
Name: SPBLB [Show more] Type: Subroutine Category: Dashboard Summary: Light up the space station indicator ("S") on the dashboard
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * KS4 calls SPBLB * NWSPS calls SPBLB * RES2 calls SPBLB
.SPBLB LDA #&20 \ Set A to the low byte of the screen address of the \ space station bulb (which is at &7D20) LDX #LO(SPBT) \ Set X to the low byte of the address of the character \ definition in SPBT LDY #&7D \ Set Y to the high byte of the screen address of the \ space station bulb (which is at &7D20) \ Fall through into BULB to draw the space station bulb
Name: BULB [Show more] Type: Subroutine Category: Dashboard Summary: Draw an indicator bulb on the dashboard
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * ECBLB calls BULB

Arguments: A The low byte of the screen address of the bulb to show X The low byte of the address of the character definition of the bulb to be drawn, i.e. #LO(ECBT) for the E.C.M. bulb, or #LO(SPBT) for the space station bulb Y The high byte of the screen address of the bulb to show 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) to the address of the character definition LDX #HI(ECBT) \ of the bulb to be drawn (this assumes that ECBT and STX P+2 \ SPBT are in the same page and have the same high byte) TYA \ Set A to Y, the high byte of the screen address we \ want to write to, so now (A SC) points to the specific \ bulb's screen address 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 call
Name: ECBT [Show more] Type: Variable Category: Dashboard Summary: The character bitmap for the E.C.M. indicator bulb
Context: See this variable on its own page Variations: See code variations for this variable in the different versions References: This variable is used as follows: * BULB calls ECBT * ECBLB calls ECBT

The character bitmap 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 a white mode 4 pixel.
.ECBT EQUB %11111110 \ x x x x x x x x . EQUB %11111110 \ x x x x x x x x . EQUB %11100000 \ x x x . . . . . . \ x x x x x x x x . \ x x x x x x x x . \ x x x . . . . . . \ x x x x x x x x . \ x x x x x x x x .
Name: SPBT [Show more] Type: Variable Category: Dashboard Summary: The bitmap definition for the space station indicator bulb
Context: See this variable on its own page Variations: See code variations for this variable in the different versions References: This variable is used as follows: * SPBLB calls SPBT

The bitmap definition for the space station indicator's "S" bulb that gets displayed on the dashboard. Each pixel is a white mode 4 pixel.
.SPBT EQUB %11111110 \ x x x x x x x x . EQUB %11111110 \ x x x x x x x x . EQUB %11100000 \ x x x . . . . . . EQUB %11111110 \ x x x x x x x x . EQUB %11111110 \ x x x x x x x x . EQUB %00001110 \ . . . . . x x x . EQUB %11111110 \ x x x x x x x x . EQUB %11111110 \ x x x x x x x x .
Name: MSBAR [Show more] Type: Subroutine Category: Dashboard Summary: Draw a specific indicator in the dashboard's missile bar
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * ABORT2 calls MSBAR * Main flight loop (Part 3 of 16) calls MSBAR * msblob calls MSBAR

Each indicator is a rectangle that's 6 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 status of the missile indicator: * &04 = black (no missile) * &11 = black "T" in white square (armed and locked) * &0D = black box in white square (armed) * &09 = white square (disarmed) Returns: X X is preserved Y Y is set to 0
.MSBAR TXA \ Store X on the stack, so we can preserve it across PHA \ the call to the subroutine ASL A \ Set T = X * 8 ASL A ASL A STA T LDA #209 \ Set SC = &80 + 32 + 49 - T SBC T \ = &80 + 32 + 48 + 1 - (X * 8) STA SC \ \ The &80 part comes from the fact that the character \ row containing the missile starts at address &7D80, \ and the low byte of this is &80 \ \ The 32 part comes from the 32-byte blank border to \ the left of the screen \ \ And the 48 part is from character block 7, which is \ the character block containing the missile indicators \ So the low byte of SC(1 0) contains the row address \ for the rightmost missile indicator, made up as \ follows: \ \ * &80 + 32 as described above \ \ * 48 (character block 7, as byte #7 * 8 = 48), 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 #&7D \ Set the high byte of SC(1 0) to &7D, the high byte of STA SCH \ &7D80, which is the start of the character row that \ contains the missile indicators (i.e. the bottom row \ of the screen) TYA \ Set X to the indicator status that was passed to the TAX \ subroutine, so we can use it below as an index into \ the MDIALS table when fetching the bitmap to set the \ missile indicator to LDY #5 \ We now want to draw this line five times, so set a \ counter in Y .MBL1 LDA MDIALS,X \ Fetch the X-th bitmap from the MDIALS table 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) DEX \ Decrement the bitmap counter for the next row DEY \ Decrement the counter for the next row BNE MBL1 \ Loop back to MBL1 if have more rows to draw PLA \ Restore X from the stack, so that it's preserved TAX RTS \ Return from the subroutine
Name: MDIALS [Show more] Type: Variable Category: Dashboard Summary: The missile indicator bitmaps for the monochrome dashboard
Context: See this variable on its own page References: This variable is used as follows: * MSBAR calls MDIALS
.MDIALS EQUB %00000000 \ No missile (black) EQUB %00000000 EQUB %00000000 EQUB %00000000 EQUB %00000000 EQUB %11111100 \ Disarmed (white square) EQUB %11111100 \ EQUB %11111100 \ Shares the first row from the next indicator EQUB %11111100 \EQUB %11111100 EQUB %11111100 \ Armed (black box in white square) EQUB %10000100 \ EQUB %10110100 \ Shares the first row from the next indicator EQUB %10000100 \EQUB %11111100 EQUB %11111100 \ Armed and locked (black "T" in white square) EQUB %11000100 EQUB %11101100 EQUB %11101100 EQUB %11111100
Name: PROJ [Show more] Type: Subroutine Category: Drawing ships Summary: Project the current ship onto the screen Deep dive: Extended screen coordinates
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PLANET calls PROJ * SHPPT calls PROJ * CIRCLE calls entry point RTS2

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) 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 Other entry points: RTS2 Contains an RTS
.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 space view, 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 space view, so this converts the STA K4+1 \ space x-coordinate into a screen y-coordinate CLC \ Clear the C flag to indicate success .RTS2 RTS \ Return from the subroutine
Name: PL2 [Show more] Type: Subroutine Category: Drawing planets Summary: Remove the planet from the screen
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PLANET calls PL2 * PLANET calls entry point PL2-1 * PROJ calls entry point PL2-1

Other entry points: PL2-1 Contains an RTS
.PL2 JMP WPLS2 \ This is the planet, so jump to WPLS2 to remove it from \ screen, returning from the subroutine using a tail \ call
Name: PLANET [Show more] Type: Subroutine Category: Drawing planets Summary: Draw the planet
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LL9 (Part 1 of 12) calls PLANET

Arguments: INWK The planet's ship data block
.PLANET LDA TYPE \ If bit 0 of the ship type is set, then this is 129, LSR A \ which is the placeholder used to denote there is no BCS PL2-1 \ space station, so return from the subroutine (as PL2-1 \ contains an RTS) LDA INWK+8 \ Set A = z_sign (the highest byte in the planet's \ coordinates) BMI PL2 \ If A is negative then the planet 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 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 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 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 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'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's radius, reduced by \ the actual distance to the planet. 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 JSR WPLS2 \ Call WPLS2 to remove the planet from the screen JMP CIRCLE \ Jump to CIRCLE to draw the planet (which is just a \ simple circle) and return from the subroutine using \ a tail call
Name: CIRCLE [Show more] Type: Subroutine Category: Drawing circles Summary: Draw a circle for the planet Deep dive: Drawing circles
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PLANET calls CIRCLE

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
.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 #9 \ If the radius < 9, skip to PL89 BCC PL89 LSR A \ Halve A so A = 4 .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 size
Name: CIRCLE2 [Show more] Type: Subroutine Category: Drawing circles Summary: Draw a circle (for the planet or chart) Deep dive: Drawing circles
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * HFS2 calls CIRCLE2 * TT128 calls CIRCLE2

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 cleared
.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 subroutine
Name: WPLS2 [Show more] Type: Subroutine Category: Drawing planets Summary: Remove the planet from the screen Deep dive: The ball line heap
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PL2 calls WPLS2 * PLANET calls WPLS2

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.
.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, returning from the subroutine \ using a tail call 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
Name: WP1 [Show more] Type: Subroutine Category: Drawing planets Summary: Reset the ball line heap
Context: See this subroutine on its own page References: This subroutine is called as follows: * WPLS2 calls WP1
.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 subroutine
Name: CHKON [Show more] Type: Subroutine Category: Drawing circles Summary: Check whether any part of a circle appears on the extended screen
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * CIRCLE calls CHKON

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)
.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 subroutine
Name: PL21 [Show more] Type: Subroutine Category: Drawing planets Summary: Return from a planet-drawing routine with a failure flag
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * CHKON calls PL21 * PLS6 calls PL21

Set the C flag and return from the subroutine. This is used to return from a planet-drawing routine with the C flag indicating an overflow in the calculation.
.PL21 SEC \ Set the C flag to indicate an overflow RTS \ Return from the subroutine
Name: PLS6 [Show more] Type: Subroutine Category: Drawing planets Summary: Calculate (X K) = (A P) / (z_sign z_hi z_lo)
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PROJ calls PLS6 * CHKON calls entry point PL44

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'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
.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) 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 subroutine
Name: TT17 [Show more] Type: Subroutine Category: Keyboard Summary: Scan the keyboard for cursor key movement
Context: See this subroutine on its own page Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main game loop (Part 5 of 6) calls TT17

Scan the keyboard for cursor key movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * 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, as an integer (see above) Y Change in the y-coordinate according to the cursor keys being pressed, as an integer (see above)
.TT17 JSR DOKEY \ Scan the keyboard for flight controls and pause keys, \ and update the key logger, setting KL to the key \ pressed LDX JSTK \ If the joystick is not configured, jump down to TJ1, BEQ TJ1 \ otherwise keep going... though as the DOKEY routine \ doesn't read the ADC channels in the Electron version, \ this doesn't actually work, but instead moves the \ crosshairs in an uncontrollable way, so this is \ presumably a bug 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 A 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, so X contains the joystick roll value LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point, and fall through into TJS1 to \ set Y to the joystick pitch value (moving the stick up \ and down) .TJS1 TAY \ Store A in Y LDA #0 \ Set the result, A = 0 CPY #16 \ If Y >= 16 set the C flag, so A = A - 1 SBC #0 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 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) LDY #0 \ Set the result, Y = 0 (and we know that X is 0 as well \ as we jumped to TJ1 from above following a LDX and a \ BEQ) CMP #&18 \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&78 \ 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 #&28 \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY RTS \ Return from the subroutine
Name: ping [Show more] Type: Subroutine Category: Universe Summary: Set the selected system to the current system
Context: See this subroutine on its own page References: This subroutine is called as follows: * BR1 (Part 2 of 2) calls ping * TT102 calls ping
.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 subroutine
Save ELTE.bin
PRINT "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 "3-assembled-output/ELTE.bin", CODE_E%, P%, LOAD%