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Elite on the BBC Micro

Drawing pixels: CPIX2 [Master version]

Name: CPIX2 [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a single-height dash on the dashboard
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: * DOT calls CPIX2 * SCAN calls CPIX2

Draw a single-height mode 2 dash (1 pixel high, 2 pixels wide). Arguments: X1 The screen pixel x-coordinate of the dash A The screen pixel y-coordinate of the dash COL The colour of the dash as a mode 2 character row byte Returns: R The dash's right pixel byte
.CPIX2 STA Y1 \ Store the y-coordinate in Y1 TAY \ Store the y-coordinate in Y LDA ylookup,Y \ Look up the page number of the character row that STA SC+1 \ contains the pixel with the y-coordinate in Y, and \ store it in the high byte of SC(1 0) at SC+1 LDA X1 \ Each character block contains 8 pixel rows, so to get AND #%11111100 \ the address of the first byte in the character block ASL A \ that we need to draw into, as an offset from the start \ of the row, we clear bits 0-1 and shift left to double \ it (as each character row contains two pages of bytes, \ or 512 bytes, which cover 256 pixels). This also \ shifts bit 7 of X1 into the C flag 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 BCC P%+5 \ If the C flag is clear then skip the next two \ instructions INC SC+1 \ The C flag is set, which means bit 7 of X1 was set \ before the ASL above, so the x-coordinate is in the \ right half of the screen (i.e. in the range 128-255). \ Each row takes up two pages in memory, so the right \ half is in the second page but SC+1 contains the value \ we looked up from ylookup, which is the page number of \ the first memory page for the row... so we need to \ increment SC+1 to point to the correct page CLC \ Clear the C flag 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 bit 1 of X1 to bit 1 of X. X will now be either AND #%00000010 \ 0 or 2, and will be double the pixel number in the TAX \ character row for the left pixel in the dash (so 0 \ means the left pixel in the 2-pixel character row, \ while 2 means the right pixel) LDA CTWOS,X \ Fetch a mode 2 1-pixel byte with the pixel position AND COL \ at X/2, 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+2,X \ Fetch a mode 2 1-pixel byte with the pixel position \ at (X+1)/2, so we can draw the right pixel of the dash BPL CP1 \ The CTWOS table has 2 extra rows at the end of it that \ repeat the first values, %10101010, 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 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 CTWOS+2,X \ Refetch the mode 2 1-pixel byte, as we just overwrote \ A (the byte will still be the fifth or sixth 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 STA R \ Store the dash's right pixel byte in R 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