Routines
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30004: Calculate Attribute Address
Used by the routines at WriteAttributeBlock, ColourShelf, MenuWriteText, ScoreLabelFlash, PrintStringColour and ColourSprite.
Converts the pixel co-ordinates to the address of the attribute buffer byte for the character cell containing them.
Input
HL Pixel address co-ordinates
Output
HL Attribute buffer address
AttributeAddress 30004 LD A,L Fetch the horizontal co-ordinate.
30005 RRCA Divide by 8 (to get the column).
30006 RRCA
30007 RRCA
30008 AND %00011111 Keep only bits 0-4 (0-31 the range of the columns).
30010 LD L,A Store this back in L.
30011 LD A,H Fetch the vertical co-ordinate.
30012 RLCA Multiply by 4 (the row is the vertical co-ordinate divided by 8 and there are 32 columns in each row).
30013 RLCA
30014 LD C,A Store this in C temporarily.
30015 AND %11100000 Keep only bits 5-7.
30017 OR L Combine with the column in L.
30018 LD L,A Store this back in L.
30019 LD A,C Fetch the stored value from C.
30020 AND %00000011 Keep only bits 0-1.
30022 OR %01011000 Set the MSB of the attribute buffer address (88). This ensures that the address is at least 22528.
30024 LD H,A Store this in H.
30025 RET Return.
View the equivalent code in;
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