Routines
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8B93: Plot a pixel
Sets the pixel at (D,E) and gives its attribute cell the ink colour held at 8C2C, keeping the cell's paper colour. If the ink would be the same as the paper (and so invisible), the ink is changed to its complement (XOR $38 before shifting).
8B93 PUSH HL
8B94 CALL $8BBC HL = the screen byte, A = the pixel's bit
8B97 PUSH AF
8B98 PUSH HL
8B99 LD A,H Work out the attribute address from the screen address
8B9A AND $18
8B9C RRCA
8B9D RRCA
8B9E RRCA
8B9F ADD A,$58
8BA1 LD H,A
8BA2 LD A,(HL) Keep only the cell's paper colour
8BA3 AND $38
8BA5 LD (HL),A
8BA6 LD A,($8C2C) A = the drawing colour, in the paper position
8BA9 RLCA
8BAA RLCA
8BAB RLCA
8BAC CP (HL) Same as the paper?
8BAD JR NZ,$8BB1
8BAF XOR $38 Yes: use its complement so it shows
8BB1 RRCA Back into the ink position
8BB2 RRCA
8BB3 RRCA
8BB4 OR (HL) Combine with the paper...
8BB5 LD (HL),A ...and store it
8BB6 POP HL
8BB7 POP AF A = the pixel's bit
8BB8 OR (HL) Set the pixel
8BB9 LD (HL),A
8BBA POP HL
8BBB RET
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