Routines
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8506: Calculate Attribute Address
Used by the routines at MenuWriteText, 8733, 890A, PrintStringColour, 8C01, 8C3E, 8C5F, LaserBeam_Animate, 9EDC, A880 and F662.
Input
HL Pixel address co-ordinates
Output
HL Attribute buffer address
Converts a given pixel address to the associated attribute buffer address.
AttributeAddress 8506 PUSH BC Stash BC on the stack.
8507 LD A,L Horizontal co-ordinate.
8508 RRCA Divide by 08.
8509 RRCA
850A RRCA
850B AND %00011111 Keep only bits 0-4 (00-1F / minimum-maximum horizontal screen values).
850D LD L,A Store this back in L.
850E LD A,H Vertical co-ordinate.
850F RLCA Multiply by 04.
8510 RLCA
8511 LD C,A Store this in C temporarily.
8512 AND %11100000 Keep only bits 5-7.
8514 OR L Set the bits from L.
8515 LD L,A L=A.
8516 LD A,C Fetch the stored value from C.
8517 AND %00000011 Keep only bits 0-1.
8519 OR %01011000 Set MSB of the attribute buffer 58. This ensures our value is >= 5800.
851B LD H,A Store this back in H.
851C POP BC Restore BC from the stack.
851D RET Return.
View the equivalent code in;
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