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Elite on the BBC Micro

Text: CHPR [Elite-A, Parasite]

Name: CHPR [Show more] Type: Subroutine Category: Text Summary: Print a character at the text cursor by sending a write_xyc command to the I/O processor Deep dive: Drawing text
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * GTDRV calls CHPR * TT26 calls CHPR * confirm calls CHPR

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
.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 LDA YC \ Fetch YC, the y-coordinate (row) of the text cursor 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 LDA #&8E \ Send command &8E to the I/O processor: JSR tube_write \ \ write_xyc(x, y, char) \ \ which will draw the text character in char at column x \ and row y LDA XC \ Send the first parameter to the I/O processor: JSR tube_write \ \ * x = XC LDA YC \ Send the second parameter to the I/O processor: JSR tube_write \ \ * y = YC TYA \ Send the third parameter to the I/O processor: JSR tube_write \ \ * char = the character in Y 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 .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