From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jonathan Larmour <jlarmour@redhat.co.uk>
To: javelina <javelina@home.com>, ecos-discuss@sourceware.cygnus.com
Subject: Re: [ECOS] twothreads.c not working correctly on ARM AEB1
Date: Sun, 02 Jul 2000 15:53:00 -0000
Message-id: <395FC7D0.1630D628@redhat.co.uk>
References: <3945AF98.2C962EAD@home.com> <39464D44.A31E032@redhat.co.uk> <3949A6A1.E2B8765C@home.com> <3949A99E.155337F7@redhat.co.uk> <39554E7A.D19FF193@home.com> <395621FA.C551538F@redhat.co.uk> <39582AA3.43ED0EF6@home.com> <39586C8C.49F02D4B@redhat.co.uk> <395F84B1.269DB524@home.com>
X-SW-Source: 2000-07/msg00002.html

[ stuff about twothreads delay not being computed correctly, traced to
dodgy % behaviour ]

javelina wrote:
>
> As a sanity check on the bad behavior of the % operator, I wrote a tiny program to
> isolate the problem
> 
>//////////////////////////////////////////////
> 
> #include <stdio.h>
> 
> int main(void)
> {
>    int i;
>    int m;
>    for (i=0; i < 21; i++) {
>        m = i % 5;
>        printf("%d %d\n", i, m);
>    }
> }
>//////////////////////////////////////////////
> 
> The ouput looks like:
> 
> (gdb) continue
> Continuing.
> 0 0
> 1 1
> 2 2
> 3 3
> 4 4
> 5 5
> 6 6
[snip] 
> So it realy does look like a mod operator problem.  Let me know what you think.

Aha! I found it, and it's interesting. Basically, for the AEB, you *must*
compile with -mcpu=arm7di as used in the example makefile. However, you
didn't compile this program, nor twothreads with this option. Whenever I
compile the above with -mcpu=arm7di it works, and without it, it behaves as
you saw. 

The reason is that without -mcpu=arm7di, the compiler will default to
-mcpu=arm7tdmi, which assumes you have a fast multiplier unit, which the
arm7di (as used on the AEB) doesn't.

As a result the compiler uses the "smull" instruction, which fails silently
- unfortunately the ARM doesn't raise an exception when this instruction is
not supported.

So compile with -mcpu=arm7di and all will be well.

Jifl
-- 
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