A sample programming session in VICMON

[Merytsetesh]

That would only be correct with sign-magnitude representation.

Ah yes, silly me again. Thank you for your patience.

So, I just ran a quick test.

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* = 828
eor_test
	CLC
	LDA #$01
	EOR #$FF
	RTS
sbc_test
	CLC
	LDA #$01
	STA $00
	LDA #$00
	SBC $00
	RTS

Both eor_test and sbc_test have the same result, flipping the contents of the accumulator between 1 and 254, or -1. I can't shake the feeling I'm missing something about SBC, else why use it?

[Mike]

[...] why use [SBC]?

If you do the sign inversion in the same context as done in BALLPING (i.e. a given zero page address as source and target), for both methods you'll arrive at (DX := $F9):

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; ** using SBC:
 SEC
 LDA #$00
 SBC $F9
 STA $F9

7 bytes, 10 cycles

Code: Select all

; ** using EOR #$FF and 2s-complement:
 CLC
 LDA $F9
 EOR #$FF
 ADC #$01
 STA $F9

9 bytes, 12 cycles

So there ...

(Also, using SBC has the charme it's independent of the way signed integers are represented)

When the value already is in the Accumulator, then using 2s-complement is shorter and faster - and it can even spare the CLC instruction when the state of the carry flag is known at that point, doing either EOR #$FF/ADC #$01 when C is known clear or EOR #$FF/ADC #$00 when C is known set. See here.

[Merytsetesh]

When the value already is in the Accumulator, then using 2s-complement is shorter and faster - and it can even spare the CLC instruction when the state of the carry flag is known at that point, doing either EOR #$FF/ADC #$01 when C is known clear or EOR #$FF/ADC #$00 when C is known set. See here.

I read that, and I also read this. I think I get it a bit better.