From:	patrickd AT cmdnet DOT lu (DOSTERT Patrick)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: !!Absolute value with inline ASM!!
Date:	Wed, 18 Feb 1998 19:42:49 GMT
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On 18 Feb 1998 08:07:18 GMT, knakasato AT aol DOT com (KNakasato) wrote:

>>I need to get the absolute value of a registers (bx)
>>How is this done in asm.(in one command, not using cmp)
>
>There's many way, but best way might be test/jz combo that you rejected for
>some unknown
>reason.
>

[snip - some ways to abs]

unpredictable branches should be avoided where ever possible

you may want to try:
	mov ebx,eax
	sar ebx,31
	xor eax,ebx
	sub eax,ebx

which calculates eax = abs(eax) without jumping ( it destroys ebx)
( for 16 bit code, replace eax/ebx with ax,bx and 31 with 15 )

you can even insert some more instructions, as in:
	mov ebx,eax
	nop
	sar ebx,31
	nop
	xor eax,ebx
	nop
	sub eax,ebx
	nop

which takes the same amount of cycles ( 4 that is )


cu
P

" Do it only with the Best "

or, in C, for those who don't read asm:

int MyAbs(int n)
{
	return  (n ^ (n >> 31)) - (n >> 31);
}

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