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Drawing pixels: PIX

[BBC Micro cassette version, Loader]

Name: PIX [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a single pixel at a specific coordinate
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: * PLL1 (Part 1 of 3) calls PIX * PLL1 (Part 2 of 3) calls PIX * PLL1 (Part 3 of 3) calls PIX

Draw a pixel at screen coordinate (X, -A). The sign bit of A gets flipped before drawing, and then the routine uses the same approach as the PIXEL routine in the main game code, except it plots a single pixel from TWOS instead of a two pixel dash from TWOS2. This applies to the top part of the screen (the monochrome mode 4 space view). See the PIXEL routine in the main game code for more details.
Arguments: X The screen x-coordinate of the pixel to draw A The screen y-coordinate of the pixel to draw, negated
.PIX TAY \ Copy A into Y, for use later EOR #%10000000 \ Flip the sign of A LSR A \ Set ZP+1 = &60 + A >> 3 LSR A LSR A ORA #&60 STA ZP+1 TXA \ Set ZP = (X >> 3) * 8 EOR #%10000000 AND #%11111000 STA ZP TYA \ Set Y = Y AND %111 AND #%00000111 TAY TXA \ Set X = X AND %111 AND #%00000111 TAX LDA TWOS,X \ Fetch a pixel from TWOS and OR it into ZP+Y ORA (ZP),Y STA (ZP),Y RTS \ Return from the subroutine