Date: Sun, 2 Nov 1997 20:31:56 -0800 (PST)
Message-Id: <199711030431.UAA13674@adit.ap.net>
Mime-Version: 1.0
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To: LEGER Patrick <leger_v AT bluewin DOT ch>, djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: global variable
Precedence: bulk

At 11:04  11/2/1997 +0100, LEGER Patrick wrote:
>Here's a sample code :
>
>static int t;
>...
>void function(int x)
>{
>  asm("pusha
>       movw $_x,%eax
>       popa");
>}
>After that the compiler say that "x" is unknown.
>But if I load x in t as static and globale variable and put it into %eax
>, the program runs.
Correct. And when you think about it, the reason is obvious. A global or
static variable is given an assembler symbol, so you can access it that way.
But a local variable is accessed only as an offset in the stack frame, and
it doesn't have a name in the assembly code. However, read on...
>Is there a method to use directly the argument without load them in a
>globale variable ?
Yes. You use GCC's Extended Asm feature. See FAQ section 18.13 for pointers
to more information. Here's an example of how you'd do what you want:

void function(int x)
{
   asm("pusha
        movw %0,%%eax      # Note double percent sign!
        popa" : /* no output */ : "g" (x)); /* specifies an input operand */
}

I'm not sure of the purpose of this code, since you store a value in eax
which is immediately overwritten by the `popa'. I guess it was just a
trivial example.


Nate Eldredge
eldredge AT ap DOT net