Routines
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D848: Calculate Attribute Address
Used by the routines at PrintText and DisplayObjectsCarried.
Finds the address in the attributes of the character square that holds a pixel.
Input
H Pixel Y coordinate (from the bottom of the screen)
L Pixel X coordinate
Output
DE Address of the attribute
Convert the pixel position into a character-square address.
CalculateAttributeAddress D848 PUSH HL Stash HL on the stack.
D849 LD A,H Invert the pixel Y coordinate, to get the line from the top.
D84A CPL
D84B LD H,A
D84C SRL H Move the line into the low bits of the high byte.
D84E SRL H
D850 SRL H
D852 SRL H
D854 RR L Move the lowest bit of the line into the low byte.
D856 SRL H Move the next bit.
D858 RR L
D85A SRL H Move the next bit, so that HL holds the number of the square.
D85C RR L
D85E LD DE,$5700 Point DE at the attributes of the screen, less one page.
D861 ADD HL,DE Add the two.
D862 EX DE,HL Put the address in DE.
D863 POP HL Restore HL from the stack.
D864 RET Return.
View the equivalent code in;
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