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Elite on the BBC Micro

Text: CHPR [Disc version, Docked]

Name: CHPR [Show more] Type: Subroutine Category: Text Summary: Print a character at the text cursor by poking into screen memory Deep dive: Drawing text
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * GTDRV calls CHPR * S% calls CHPR * TT26 calls CHPR * BULB calls entry point RREN

Print a character at the text cursor (XC, YC), do a beep, print a newline, or delete left (backspace). WRCHV is set to point here by the loading process. 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) * 127 (delete the character to the left of the text cursor and move the cursor to the left) XC Contains the text column to print at (the x-coordinate) YC Contains the line number to print on (the y-coordinate) Returns: A A is preserved X X is preserved Y Y is preserved C flag The C flag is cleared Other entry points: RR3+1 Contains an RTS RREN Prints the character definition pointed to by P(2 1) at the screen address pointed to by (A SC). Used by the BULB routine
.CHPR STA K3 \ Store the A, X and Y registers, so we can restore STY YSAV2 \ them at the end (so they don't get changed by this STX XSAV2 \ routine) .RRNEW LDY QQ17 \ Load the QQ17 flag, which contains the text printing \ flags INY \ If QQ17 = 255 then printing is disabled, so jump to BEQ RR4 \ RR4, which doesn't print anything, it just restores \ the registers and returns from the subroutine TAY \ Set Y = the character to be printed BEQ RR4 \ If the character is zero, which is typically a string \ terminator character, jump down to RR4 to restore the \ registers and return from the subroutine BMI RR4 \ If A > 127 then there is nothing to print, so jump to \ RR4 to restore the registers and return from the \ subroutine CMP #7 \ If this is a beep character (A = 7), jump to R5, BEQ R5 \ which will emit the beep, restore the registers and \ return from the subroutine CMP #32 \ If this is an ASCII character (A >= 32), jump to RR1 BCS RR1 \ below, which will print the character, restore the \ registers and return from the subroutine CMP #10 \ If this is control code 10 (line feed) then jump to BEQ RRX1 \ RRX1, which will move down a line, restore the \ registers and return from the subroutine LDX #1 \ If we get here, then this is control code 11-13, of STX XC \ which only 13 is used. This code prints a newline, \ which we can achieve by moving the text cursor \ to the start of the line (carriage return) and down \ one line (line feed). These two lines do the first \ bit by setting XC = 1, and we then fall through into \ the line feed routine that's used by control code 10 CMP #13 \ If this is control code 13 (carriage return) then jump BEQ RR4 \ RR4 to restore the registers and return from the \ subroutine .RRX1 INC YC \ Print a line feed, simply by incrementing the row \ number (y-coordinate) of the text cursor, which is \ stored in YC BNE RR4 \ Jump to RR4 to restore the registers and return from \ the subroutine (this BNE is effectively a JMP as Y \ will never be zero) .RR1 \ If we get here, then the character to print is an \ ASCII character in the range 32-95. The quickest way \ to display text on-screen is to poke the character \ pixel by pixel, directly into screen memory, so \ that's what the rest of this routine does \ \ The first step, then, is to get hold of the bitmap \ definition for the character we want to draw on the \ screen (i.e. we need the pixel shape of this \ character). The MOS ROM contains bitmap definitions \ of the system's ASCII characters, starting from &C000 \ for space (ASCII 32) and ending with the £ symbol \ (ASCII 126) \ \ There are definitions for 32 characters in each of the \ three pages of MOS memory, as each definition takes up \ 8 bytes (8 rows of 8 pixels) and 32 * 8 = 256 bytes = \ 1 page. So: \ \ ASCII 32-63 are defined in &C000-&C0FF (page 0) \ ASCII 64-95 are defined in &C100-&C1FF (page 1) \ ASCII 96-126 are defined in &C200-&C2F0 (page 2) \ \ The following code reads the relevant character \ bitmap from the above locations in ROM and pokes \ those values into the correct position in screen \ memory, thus printing the character on-screen \ \ It's a long way from 10 PRINT "Hello world!":GOTO 10 \ Now we want to set X to point to the relevant page \ number for this character - i.e. &C0, &C1 or &C2. \ The following logic is easier to follow if we look \ at the three character number ranges in binary: \ \ Bit # 76543210 \ \ 32 = %00100000 Page 0 of bitmap definitions \ 63 = %00111111 \ \ 64 = %01000000 Page 1 of bitmap definitions \ 95 = %01011111 \ \ 96 = %01100000 Page 2 of bitmap definitions \ 125 = %01111101 \ \ We'll refer to this below LDX #&BF \ Set X to point to the first font page in ROM minus 1, \ which is &C0 - 1, or &BF ASL A \ If bit 6 of the character is clear (A is 32-63) ASL A \ then skip the following instruction BCC P%+4 LDX #&C1 \ A is 64-126, so set X to point to page &C1 ASL A \ If bit 5 of the character is clear (A is 64-95) BCC P%+3 \ then skip the following instruction INX \ Increment X \ \ By this point, we started with X = &BF, and then \ we did the following: \ \ If A = 32-63: skip then INX so X = &C0 \ If A = 64-95: X = &C1 then skip so X = &C1 \ If A = 96-126: X = &C1 then INX so X = &C2 \ \ In other words, X points to the relevant page. But \ what about the value of A? That gets shifted to the \ left three times during the above code, which \ multiplies the number by 8 but also drops bits 7, 6 \ and 5 in the process. Look at the above binary \ figures and you can see that if we cleared bits 5-7, \ then that would change 32-53 to 0-31... but it would \ do exactly the same to 64-95 and 96-125. And because \ we also multiply this figure by 8, A now points to \ the start of the character's definition within its \ page (because there are 8 bytes per character \ definition) \ \ Or, to put it another way, X contains the high byte \ (the page) of the address of the definition that we \ want, while A contains the low byte (the offset into \ the page) of the address STA P+1 \ Store the address of this character's definition in STX P+2 \ P(2 1) LDA XC \ Fetch XC, the x-coordinate (column) of the text cursor \ into A LDX CATF \ If CATF = 0, jump to RR5, otherwise we are printing a BEQ RR5 \ disc catalogue CPY #' ' \ If the character we want to print in Y is a space, BNE RR5 \ jump to RR5 \ If we get here, then CATF is non-zero, so we are \ printing a disc catalogue and we are not printing a \ space, so we drop column 17 from the output so the \ catalogue will fit on-screen (column 17 is a blank \ column in the middle of the catalogue, between the \ two lists of filenames, so it can be dropped without \ affecting the layout). Without this, the catalogue \ would be one character too wide for the square screen \ mode (it's 34 characters wide, while the screen mode \ is only 33 characters across) CMP #17 \ If A = 17, i.e. the text cursor is in column 17, jump BEQ RR4 \ to RR4 to restore the registers and return from the \ subroutine, thus omitting this column .RR5 ASL A \ Multiply A by 8, and store in SC. As each character is ASL A \ 8 pixels wide, and the special screen mode Elite uses ASL A \ for the top part of the screen is 256 pixels across STA SC \ with one bit per pixel, this value is not only the \ screen address offset of the text cursor from the left \ side of the screen, it's also the least significant \ byte of the screen address where we want to print this \ character, as each row of on-screen pixels corresponds \ to one page. To put this more explicitly, the screen \ starts at &6000, so the text rows are stored in screen \ memory like this: \ \ Row 1: &6000 - &60FF YC = 1, XC = 0 to 31 \ Row 2: &6100 - &61FF YC = 2, XC = 0 to 31 \ Row 3: &6200 - &62FF YC = 3, XC = 0 to 31 \ \ and so on LDA YC \ Fetch YC, the y-coordinate (row) of the text cursor CPY #127 \ If the character number (which is in Y) <> 127, then BNE RR2 \ skip to RR2 to print that character, otherwise this is \ the delete character, so continue on DEC XC \ We want to delete the character to the left of the \ text cursor and move the cursor back one, so let's \ do that by decrementing YC. Note that this doesn't \ have anything to do with the actual deletion below, \ we're just updating the cursor so it's in the right \ position following the deletion ADC #&5E \ A contains YC (from above) and the C flag is set (from TAX \ the CPY #127 above), so these instructions do this: \ \ X = YC + &5E + 1 \ = YC + &5F \ Because YC starts at 0 for the first text row, this \ means that X will be &5F for row 0, &60 for row 1 and \ so on. In other words, X is now set to the page number \ for the row before the one containing the text cursor, \ and given that we set SC above to point to the offset \ in memory of the text cursor within the row's page, \ this means that (X SC) now points to the character \ above the text cursor LDY #&F8 \ Set Y = &F8, so the following call to ZES2 will count \ Y upwards from &F8 to &FF JSR ZES2 \ Call ZES2, which zero-fills from address (X SC) + Y to \ (X SC) + &FF. (X SC) points to the character above the \ text cursor, and adding &FF to this would point to the \ cursor, so adding &F8 points to the character before \ the cursor, which is the one we want to delete. So \ this call zero-fills the character to the left of the \ cursor, which erases it from the screen BEQ RR4 \ We are done deleting, so restore the registers and \ return from the subroutine (this BNE is effectively \ a JMP as ZES2 always returns with the Z flag set) .RR2 \ Now to actually print the character INC XC \ Once we print the character, we want to move the text \ cursor to the right, so we do this by incrementing \ XC. Note that this doesn't have anything to do \ with the actual printing below, we're just updating \ the cursor so it's in the right position following \ the print CMP #24 \ If the text cursor is on the screen (i.e. YC < 24, so BCC RR3 \ we are on rows 1-23), then jump to RR3 to print the \ character PHA \ Store A on the stack so we can retrieve it below JSR TTX66 \ Otherwise we are off the bottom of the screen, so \ clear the screen and draw a white border PLA \ Retrieve A from the stack... only to overwrite it with \ the next instruction, so presumably we didn't need to \ preserve it and this and the PHA above have no effect LDA K3 \ Set A to the character to be printed JMP RRNEW \ Jump back to RRNEW to print the character .RR3 \ A contains the value of YC - the screen row where we \ want to print this character - so now we need to \ convert this into a screen address, so we can poke \ the character data to the right place in screen \ memory ORA #&60 \ We already stored the least significant byte \ of this screen address in SC above (see the STA SC \ instruction above), so all we need is the most \ significant byte. As mentioned above, in Elite's \ square mode 4 screen, each row of text on-screen \ takes up exactly one page, so the first row is page \ &60xx, the second row is page &61xx, so we can get \ the page for character (XC, YC) by OR'ing with &60. \ To see this in action, consider that our two values \ are, in binary: \ \ YC is between: %00000000 \ and: %00010111 \ &60 is: %01100000 \ \ so YC OR &60 effectively adds &60 to YC, giving us \ the page number that we want .RREN STA SC+1 \ Store the page number of the destination screen \ location in SC+1, so SC now points to the full screen \ location where this character should go LDY #7 \ We want to print the 8 bytes of character data to the \ screen (one byte per row), so set up a counter in Y \ to count these bytes .RRL1 LDA (P+1),Y \ The character definition is at P(2 1) - we set this up \ above - so load the Y-th byte from P(2 1), which will \ contain the bitmap for the Y-th row of the character ORA (SC),Y \ OR this value with the current contents of screen \ memory, so the pixels we want to draw are set STA (SC),Y \ Store the Y-th byte at the screen address for this \ character location DEY \ Decrement the loop counter BPL RRL1 \ Loop back for the next byte to print to the screen .RR4 LDY YSAV2 \ We're done printing, so restore the values of the LDX XSAV2 \ A, X and Y registers that we saved above and clear LDA K3 \ the C flag, so everything is back to how it was CLC RTS \ Return from the subroutine .R5 JSR BEEP \ Call the BEEP subroutine to make a short, high beep JMP RR4 \ Jump to RR4 to restore the registers and return from \ the subroutine using a tail call