RosettaCodeData/Task/Twos-complement/6502-Assembly/twos-complement-2.6502

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myVar equ $20
LDA #3
STA myVar
LDA #0
STA myVar+1 ;equivalent C: uint16_t myVar = 3;
negate:
LDA myVar+1
EOR #255
STA myVar+1
LDA myVar
EOR #255
STA myVar
CLC
ADC #1
STA myVar
;this handles the case if we started with something where the low byte was zero.
LDA myVar+1
ADC #0
STA myVar+1