base:signed_8bit_16bit_addition
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To add a signed 8-bit delta to a 16-bit value, we need to sign-extend the delta to a full 16 bits. We hold the high byte of the delta in .X and set it to $00 or $ff based on its sign:
ldx #$00 ; implied high byte of delta lda delta bpl :+ dex ; high byte becomes $ff to reflect negative delta :clc adc value ; normal 16-bit addition sta value txa adc value+1 sta value+1
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base/signed_8bit_16bit_addition.1508995836.txt.gz · Last modified: 2017-10-26 07:30 by white_flame