First post, by LSS10999
The way to enable SSE is well known and thanks to SIMD95 we now have basic SSE capability in DOS as well as DOS-based Windows (95, or maybe 3.x also).
Setting the configuration registers only takes care of enabling the use of these instructions (so SSE instructions won't #UD from that point on) but so far nothing I know has touched the subject on context switching (FXSAVE/FXRSTOR).
I'm not an expert, but from what I read it seems context switching is mostly about multithreading. Per SIMD95's documentation:
"You cannot have two or more application running with new instructions or one multi-thread application."
I'm not sure how relevant SSE context switching would be for DOS, as I don't think DOS has the concept of threads compared to Windows, but I wonder if things like TSRs or INT handlers can be considered as such...
Here's a thread on OSDev regarding to adding SSE support in an OS kernel. It's not about DOS but it might contain some useful info on performing context switching.