Routines
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34054: Calculate Attribute Address
Used by the routines at MenuWriteText, ColouriseSprite, WriteSpriteAttributes, PrintStringColour, DrawGauge, ScoreLabelFlash, UnsetFlashText, LaserBeam_Animate, Handler_RoverFrames, MoveDebrisParticle, Handler_AlienBasePlasma and ColourLevelsGreen.
Converts a pixel address into the matching attribute buffer address.
Input
HL Pixel address co-ordinates
Output
HL Attribute buffer address
Convert the pixel X/Y in HL into an attribute address.
AttributeAddress 34054 PUSH BC Stash BC on the stack.
34055 LD A,L Horizontal co-ordinate.
34056 RRCA Divide by 8.
34057 RRCA
34058 RRCA
34059 AND %00011111 Keep only bits 0-4 (0-31 / minimum-maximum horizontal screen values).
34061 LD L,A Store this back in L.
Merge the vertical co-ordinate into the low byte.
34062 LD A,H Vertical co-ordinate.
34063 RLCA Multiply by 4.
34064 RLCA
34065 LD C,A Store this in C temporarily.
34066 AND %11100000 Keep only bits 5-7.
34068 OR L Set the bits from L.
34069 LD L,A L=A.
Work out the high byte in the attribute buffer.
34070 LD A,C Fetch the stored value from C.
34071 AND %00000011 Keep only bits 0-1.
34073 OR %01011000 Set the attribute buffer MSB to 88.
34075 LD H,A Store this back in H.
34076 POP BC Restore BC from the stack.
34077 RET Return.
View the equivalent code in;
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