I'm writing assembly routines in 32-bit CPU mode, using directives .686 and .XMM so that I can utilize SSE instructions. I needed to start with register XMM1 as all-0, so I have been using PXOR xmm1,xmm1 to set it to 0. But when assembling with MASM, I noticed that in the Register window that while the general purpose registers all started with various values in them (as expected), the XMM registers (XMM0-XMM7) all started as all-0. They appear to be automatically initialized to 0. So it made me wonder if I actually need to use PXOR to set it to 0. Is this a known "feature" of the XMM registers, and if so, is it documented anywhere? I Googled but didn't find anything about it.
Thanks. / Rav
Rav,
You would have to determine that from the Intel manuals and then check if its a characteristic of the operating system. Safety says do it yourself, then you know its done.
On my machine, even ecx, esi, edi and the whole FPU are zero at the entry point. Will I rely on that? Never.
Hutch is right, if it's not explicitly documented, don't use it. There are some badly documented grey areas; for example, variables in the .DATA? section are initialised to zero, but you will have a problem finding a Micros*t doc which says so.
Thanks, hutch and jj2007. I'll go with safety and initialize it.