Routines
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9AA5: Subtract Width From Address
Used by the routines at DrawGraphic_Flipped, DrawGraphic_TransposedFlipped, DrawGraphic_RotatedRight, SetAttributes_Flipped, SetAttributes_TransposedFlipped and SetAttributes_RotatedRight.
Subtracts the width of a graphic from DE, to move back one row.
Input
B The width of the graphic
DE The address
Output
DE The address minus the width
SubtractWidthFromAddress 9AA5 LD A,E Fetch the low byte of the address.
9AA6 SUB B Subtract the width.
9AA7 LD E,A Keep the new low byte in E.
9AA8 LD A,D Fetch the high byte of the address.
9AA9 SBC A,$00 Subtract the carry.
9AAB LD D,A Keep the new high byte in D.
9AAC RET Return.
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