[SDL] Flickering and division by zero in Windows

[Andre de Leiradella]

>> 2) Is the FPU control register something I can change just by reading 
>> it, and'ing or or'ing with a mask and writing back the new value?
>>     
>
> Yes, but probably not in Lua directly. There's an x86/amd64 assembly 
> instruction for reading and writing that register (and, from C, there's 
> usually a non-standard function called _controlfp() or something similar 
> to do the same).
>
> If you can't figure out what's changing the control word, or you can't 
> stop it from changing it, forcing it back yourself will probably Just Work.
>
> --ryan.
>
>   
I was already playing with fstcw and fldcw when I received your message. 
For the records, to disable division by zero exceptions one can use

_control87(MCW_EM, EM_ZERODIVIDE);

Also for the records, here is a small assembler function that disables 
the same exception for those who doesn't have _control87 nor _controlfp:

-------------------8<-------------------
segment _TEXT use32 align=4 class=CODE

global _setzm

_setzm:
                   lea eax, [esp - 4]
                   fstcw [eax]
                   or byte [eax], 4
                   fldcw [eax]
                   ret
-------------------8<-------------------

It can be compiled with nasm.

The strange part is that no function is resetting the flag, it already 
comes zeroed when the program starts! I imagine that it must be 
something from the bootstrap code... Oh well, using _control87 at the 
beginning of the program solved the issue.

Thanks a lot Ryan for pointing me to both fldcw and _control87.

Cheers,

Andre