Mail Archives: djgpp/1997/10/04/05:34:32

From: tom burgess <tburgess AT drao DOT nrc DOT ca>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: asm xor problems
Date: Sat, 04 Oct 1997 00:44:44 -0700
Organization: BCTEL Advanced Communications
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References: <EHI7xn DOT FFs AT nonexistent DOT com>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Arthur Bredrick wrote:
>  Why do I get the following error with the assembly statement, "xorb
> variable, variable"?
> "Error:  Error:  operands given don't match any known 386 instruction"
> Is it illegal to use xor with the same variable twice?  If so, why?
> Art

The advanced RRSC (Reduced Register Set Computer :) x86 family
does not support memory to memory operations. You have to
get one of the arguments into a precious register first.
Something like (I might have the syntax wrong): 

	"movb variable1, %0"	// let GCC pick the register (%0)
	"xorb %0, variable2"	// xor variable1 into variable2

	regards, tom

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