Re: [cc65] Signed comparisions

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From: Piotr Fusik (P.Fusik_at_elka.pw.edu.pl)
Date: 2003-10-28 12:08:05 

Hi,

On Mon, 27 Oct 2003, Ullrich von Bassewitz wrote:

> > 2. Signed comparisions can be made unsigned. :-)
> [...]
> > result2 = (unsigned) (a ^ 0x8000) < (unsigned) (b ^ 0x8000);
> > In cases when we didn't use CPX this gives penalty of just one EOR #$80
> > comparing with the old incorrect code or unsigned comparisions.
>
> The problem is that the high byte is in X, so it's not only the EOR that is
> needed but the value has to moved into A for doing so.

That's right. I stated that it won't work that good for cases where CPX
is used. But generally 16-bit compares are like this:
	cmp	#<value
	txa
	sbc	#>value
So, let's just put an EOR #$80 between TXA and SBC.

> And, since A contains the low byte, it has to be saved somewhere.

I thought that the result of comparision replaces the current AX value?

Bye,
Piotr

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