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

[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.


>
> ATB,
>
> Mark.
>
> --
> Mark Cave-Ayland - Senior Technical Architect
> PostgreSQL - PostGIS
> Sirius Corporation plc - control through freedom
> http://www.siriusit.co.uk
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>
> Sirius Labs: http://www.siriusit.co.uk/labs
>
> --
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>

William Hahne
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