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

William Hahne will07c5 at gmail.com
Wed Aug 10 19:35:37 CEST 2011


On Wed, Aug 10, 2011 at 11:54 AM, Andreas Färber <andreas.faerber at web.de>wrote:

> 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 <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
>
>
That makes sense. Here is a patch with an appropriate #ifdef added.

Index: kernel/forth.c
===================================================================
--- kernel/forth.c      (revision 1046)
+++ kernel/forth.c      (working copy)
@@ -1157,6 +1157,17 @@
 {
        const ucell b = POP();
        const ducell a = DPOP();
+
+#if defined(CONFIG_PPC) || defined(CONFIG_PPC64)
+       /* PPC OF provides a result of 0 after a division by zero
+           rather than an exception */
+        if (b == 0) {
+            PUSH(0);
+            DPUSH(0);
+            return;
+       }
+#endif
+
 #ifdef NEED_FAKE_INT128_T
         if (a.hi != 0) {
             fprintf(stderr, "mudivmod called (0x%016llx %016llx /
0x%016llx)\n",



> --
> OpenBIOS                 http://openbios.org/
> Mailinglist:  http://lists.openbios.org/**mailman/listinfo<http://lists.openbios.org/mailman/listinfo>
> Free your System - May the Forth be with you
>

William Hahne
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openbios.org/pipermail/openbios/attachments/20110810/b0f9c592/attachment-0001.html>


More information about the OpenBIOS mailing list