Routines
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46006: Calculate Attribute Address
Used by the routines at DisplayObjectsCarried and PrintText.
Finds the address in the attribute file of the colour square that holds a pixel. The y co-ordinate counts up from the bottom of the screen. The result is based on 22272 rather than 22528 as the lowest eight character rows are below the screen.
Input
H Pixel y co-ordinate, counting up from the bottom of the screen
L Pixel x co-ordinate
Output
DE Address in the attribute file
Turn the pixel position into a character offset.
CalculateAttributeAddress 46006 PUSH HL Stash HL on the stack.
46007 LD A,H Invert the y co-ordinate.
46008 CPL
46009 LD H,A
46010 SRL H Shift the inverted y co-ordinate right four times.
46012 SRL H
46014 SRL H
46016 SRL H
46018 RR L Rotate the last bit shifted out into the top of L, shifting the x co-ordinate right by one.
46020 SRL H Shift H again, and rotate the bit shifted out into L.
46022 RR L
46024 SRL H Shift H once more, and rotate the bit shifted out into L. This leaves HL as the character row times 32 plus the column.
46026 RR L
Add the attribute file base.
46028 LD DE,22272 Set DE to 22272, the base of the attribute file less 256.
46031 ADD HL,DE Add the offset to get the address of the attribute.
46032 EX DE,HL Keep the attribute address.
46033 POP HL Restore HL from the stack.
46034 RET Return.
View the equivalent code in;
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