ppc assembler help needed

ppc assembler help needed

[Andreas Tobler]

Hi,

I have some problems understanding the following code snippet. It is
from misc.S in kernel 2.3.

The function call is like this:

void _insl_ns(volatile u32 *port, u32 *buf, int nl)

_GLOBAL(_insl_ns)
        cmpw  0,r5,0
        mtctr   r5
        subi    r4,r4,4
        blelr-
00:     lwz     r5,0(r3)
        eieio
        stwu    r5,4(r4)
        bdnz    00b
        blr

Am I right with port = r3, buf = r4, nl = r5?
If so, cmpw 0,r5,0 compares the content of r5, in my case nl, with zero
and stores the result in CR0?

What does blelr- exactly do? 'branch if less than or equal' according to
the 'Programming Environment for 32bit uPs' from Motorola.
Commenting out the cmpw and the blelr- line as in kernel-2.2, then the
function works. The values from nl are in the range from 10 to 20 (int).
So the the blelr- shouldn't happen.

The function in C looks a bit like: (Thanks to Paul)

void _insl_ns(volatile u32 *port, u32 *buf, int nl)
{
        if (nl <= 0)
                return;
        do {
                *buf++ = *port;
                eieio();
        } while (--nl != 0);
}

Thanks for any explanation

Andreas

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Re: ppc assembler help needed

[Franz Sirl]

At 17:54 15.05.00, Andreas Tobler wrote:

>Hi,
>
>I have some problems understanding the following code snippet. It is
>from misc.S in kernel 2.3.
>
>The function call is like this:
>
>void _insl_ns(volatile u32 *port, u32 *buf, int nl)
>
>_GLOBAL(_insl_ns)
>         cmpw  0,r5,0
>         mtctr   r5
>         subi    r4,r4,4
>         blelr-
>00:     lwz     r5,0(r3)
>         eieio
>         stwu    r5,4(r4)
>         bdnz    00b
>         blr
>
>Am I right with port = r3, buf = r4, nl = r5?
>If so, cmpw 0,r5,0 compares the content of r5, in my case nl, with zero
>and stores the result in CR0?

No, cmpw 0,r5,0 compares r5 to r0, use cmpwi 0,r5,0 to make a signed
compare of r5 against 0

Franz.


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Re: ppc assembler help needed

[Andreas Tobler]

Hoi Franz,

Franz Sirl wrote:
> No, cmpw 0,r5,0 compares r5 to r0, use cmpwi 0,r5,0 to make a signed
> compare of r5 against 0

This works, thanks. The next question is automatically in the queue:

all functions, insX/outsX in misc.S use this cmpw 0,r5,0, shouldn't it
be a cmpwi 0,r5,0 then?

Thanks,
Andreas

BTW, does it mind which CPU is behind? In my case it's a 601.

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Re: ppc assembler help needed

[David Edelsohn]

>>>>> Andreas Tobler writes:

Andreas> Franz Sirl wrote:
>> No, cmpw 0,r5,0 compares r5 to r0, use cmpwi 0,r5,0 to make a signed
>> compare of r5 against 0

Andreas> This works, thanks. The next question is automatically in the queue:

Andreas> all functions, insX/outsX in misc.S use this cmpw 0,r5,0, shouldn't it
Andreas> be a cmpwi 0,r5,0 then?

	Presumably all of the assembly functions want to compare against
an immediate value and should use cmpwi.

Andreas> BTW, does it mind which CPU is behind? In my case it's a 601.

	cmpwi is valid for all PowerPC processors, including 601.

David

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