Routines
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39890: Calculate Attribute Address
Used by the routines at MenuWriteText, HandlerFoodItem, SetKeyPositions, SetEntityAttributes, PrintStringColour and DrawSidePanelAttrs.
Input
HL Pixel address co-ordinates
Output
HL Attribute buffer address
Converts a given pixel address to the associated attribute buffer address.
AttributeAddress 39890 PUSH BC Stash BC on the stack.
39891 LD A,L Horizontal co-ordinate.
39892 RRCA Divide by 8.
39893 RRCA
39894 RRCA
39895 AND %00011111 Keep only bits 0-4 (0-31 / minimum-maximum horizontal screen values).
39897 LD L,A Store this back in L.
39898 LD A,H Vertical co-ordinate.
39899 RLCA Multiply by 4.
39900 RLCA
39901 LD C,A Store this in C temporarily.
39902 AND %11100000 Keep only bits 5-7.
39904 OR L Set the bits from L.
39905 LD L,A L=A.
39906 LD A,C Fetch the stored value from C.
39907 AND %00000011 Keep only bits 0-1.
39909 OR %01011000 Set MSB of the attribute buffer 88. This ensures our value is >= 22528.
39911 LD H,A Store this back in H.
39912 POP BC Restore BC from the stack.
39913 RET Return.
View the equivalent code in;
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