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Drawing pixels: CPIX2

[BBC Micro cassette version]

Name: CPIX2 [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a single-height dash on the dashboard Deep dive: Drawing colour pixels in mode 5
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: * CPIX4 calls CPIX2 * DOT calls CPIX2

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