[OpenBIOS] [PATCH 9/10] Divide by zero check

[Andreas Färber]

Am 10.08.2011 um 17:48 schrieb William Hahne:

> On Wed, Aug 10, 2011 at 6:00 AM, Mark Cave-Ayland <mark.cave-ayland at siriusit.co.uk 
> > wrote:
>> On 09/08/11 22:55, William Hahne wrote:
>>
>>> It is possible for BootX to purposefully divide by zero expecting  
>>> 0 as
>>> the result.
>>>
>>> Index: kernel/forth.c
>>> ===================================================================
>>> --- kernel/forth.c (revision 1041)
>>> +++ kernel/forth.c (working copy)
>>> @@ -1157,6 +1157,12 @@
>>>  {
>>> const ucell b = POP();
>>> const ducell a = DPOP();
>>> +
>>> + if (b == 0) { // can't divide by zero
>>> +            PUSH(0);
>>> +            DPUSH(0);
>>> +            return;
>>> + }
>>>  #ifdef NEED_FAKE_INT128_T
>>>          if (a.hi != 0) {
>>>              fprintf(stderr, "mudivmod called (0x%016llx %016llx /
>>> 0x%016llx)\n",
>>
>> There is something wrong here: nothing will purposefully divide by  
>> zero as it will invoke a trap somewhere. Again, I think this is  
>> hiding another bug somewhere and is not necessarily the correct fix.
>
> You would be surprised then, if you type "0 0 /" into a OF mac you  
> will get 0. I thought it was a bug as well at first but after  
> carefully looking at the source code for BootX I found that it  
> wasn't. This is probably just a peculiarity of Apple's OF  
> implementation.

I'd rather think this is an architectural difference: On ppc, divwo  
sets the OV bit in XER but divw doesn't, PowerISA 2.06B saying that  
the result of the division is undefined in both cases.

On POWER5 I do get zero as well, so it's not just Apple.

0 > 0 0 /  ok
1 > . 0  ok

I'd suggest to stick the new code fragment into an appropriate ppc  
#ifdef if sparc does it differently.

Andreas