linux-kernel - Re: Inline assembly queries [2]

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Date:	Sat, 4 Jul 2009 01:35:50 +0530
From:	kernel mailz <kernelmailz@...glemail.com>
To:	Brad Boyer <flar@...andria.com>
Cc:	linuxppc-dev@...abs.org, gcc-help@....gnu.org,
	linux-kernel@...r.kernel.org
Subject: Re: Inline assembly queries [2]

Hi Brad,
Thanks for responding.
My query was more on %U1%X1, I guess it is specifying U and/or X for %1 right ?
what does U/X stand for (is it similar to u - unsigned and x for a hex address)
are there any more literals like U/X/...

-Manish

On Fri, Jul 3, 2009 at 11:10 PM, Brad Boyer<flar@...andria.com> wrote:
> On Fri, Jul 03, 2009 at 12:14:41PM +0530, kernel mailz wrote:
>> Thanks for responding to my previous mail. A few more queries
>>
>> a. What is the use of adding format specifiers in inline assembly
>> like
>> asm volatile("ld%U1%X1 %0,%1":"=r"(ret) : "m"(*ptr) : "memory");
>
> The format specifiers limit which registers or addressing modes will
> be chosen to access that parameter. For example, if you're using an
> instruction that treats r0 as a literal 0 value, you can't use the
> "r" specifier for that or you may see problems if it happens to allocate
> r0 for that particular argument. For memory access, the "m" lets you
> use any of the normal load/store patterns (which is why this particular
> choice also requires the "%U1%X1" part to allow changing the instruction).
> The system was setup for an older style chip like x86 or 68k with many
> restrictions on which registers can be used where and large numbers of
> different addressing modes for accessing memory. It's a little clumsy
> for ppc by comparison to most other chips, but it's a fundamental part
> of inline assembly processing in gcc.
>
>> b. using m or Z  with a memory address. I tried replacing m/Z but no change
>> Is there some guideline ?
>> gcc documentation says Z is obsolete. Is m/Z replaceable ?
>
> No idea. I don't remember ever seeing 'Z' used in anything. Maybe somebody
> else remembers what it used to mean.
>
>        Brad Boyer
>        flar@...andria.com
>
>
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