Routines
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41328: Find Attribute Address
Used by the routines at PrintText, UpdateStarLayer, DrawShipTop, FillAttributes, CopyAttributes and ColourSegment.
Finds the address in the attribute file of a pixel position. The vertical position is the number of the row of pixels, counted from the top of the screen.
Input
H Vertical position
L Horizontal position
Output
HL Attribute address
AttributeAddress 41328 PUSH BC Stash BC on the stack.
41329 LD A,L Fetch the horizontal position.
41330 RRC A Divide it by eight.
41332 RRC A
41334 RRC A
41336 AND %00011111 Keep the five bits of the column.
41338 LD L,A Store it in L.
41339 LD A,H Fetch the vertical position.
41340 RLC A Multiply it by four.
41342 RLC A
41344 LD C,A Keep a copy.
41345 AND %11100000 Keep the bits that give the row of the cell.
41347 OR L Combine them with the column.
41348 LD L,A Store the low byte of the address.
41349 LD A,C Fetch the multiplied vertical position.
41350 AND %00000011 Keep the bits that give the third of the screen.
41352 OR %01011000 Add the address of the attribute file.
41354 LD H,A Store the high byte of the address.
41355 POP BC Bring BC back off the stack.
41356 RET Return.
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