Routines
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44512: Calculate Attribute Address
Used by the routines at MenuWriteText, ColourEntity, Set_Area_Attributes, PrintStringColour, Draw_Ceiling_Picture, Draw_Left_Wall_Picture, Draw_Top_Left_Corner_Picture, Draw_Line, Handle_Stalactite and Handle_Rock_Formation.
Converts a pixel position into the address of the matching cell of the attribute buffer.
Input
HL Position (L is the x position, H is the y position)
Output
HL Attribute buffer address
AttributeAddress 44512 PUSH BC Stash BC on the stack.
44513 LD A,L Fetch the x position.
44514 RRCA Divide it by 8.
44515 RRCA
44516 RRCA
44517 AND 31 Keep only five bits.
44519 LD L,A Store the column in L.
44520 LD A,H Fetch the y position.
44521 RLCA Multiply it by 4.
44522 RLCA
44523 LD C,A Store the result in C.
44524 AND 224 Keep only the bits that give the low part of the row.
44526 OR L Combine them with the column.
44527 LD L,A Store the low byte of the address in L.
44528 LD A,C Fetch the multiplied y position.
44529 AND 3 Keep only the bits that give the high part of the row.
44531 OR 88 Add the start of the attribute buffer.
44533 LD H,A Store the high byte of the address in H.
44534 POP BC Restore BC from the stack.
44535 RET Return.
View the equivalent code in;
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