Routines
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E53A: Calculate Buffer Address
Used by the routines at DrawStripGraphic, DrawWallStrip and DrawSprite.
Works out the address in the screen buffer of a position in the window. The buffer is 24 bytes wide.
Input
H Y position (from 48 at the first row of the buffer)
L X position
Output
HL Address in the screen buffer
Work out the column and 24 times the row.
CalculateBufferAddress E53A PUSH BC Stash BC on the stack.
E53B SRL L Divide the X position by eight.
E53D SRL L
E53F SRL L
E541 LD A,L Keep the column, and start with the Y position.
E542 LD L,H
E543 LD H,$00
E545 ADD HL,HL Multiply the Y position by eight.
E546 ADD HL,HL
E547 ADD HL,HL
E548 LD C,L Keep the result.
E549 LD B,H
E54A ADD HL,HL Multiply by two more, and add it to get 24 times the Y position.
E54B ADD HL,BC
Add the column and the address of the buffer.
E54C LD C,A Add the X position divided by eight.
E54D LD B,$00
E54F ADD HL,BC Add the column.
E550 LD BC,$DF02 Add the address of the buffer.
E553 ADD HL,BC
E554 POP BC Restore BC from the stack and return.
E555 RET
View the equivalent code in;
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