64bit registers on 32bit binaries: At what an extend are they used? [dependence to OS, Machine considered]

Question

Any tips to programming would be appreciated.

Answer 1

MMX, SSE and 3DNow use 64 or 128 bits registers

However, you won't "program" them yourself unless you're really low level in assembler or writing a compiler. It's transparent to pretty much everyone one

Answer 2

With x86-64 the general purpose registers are 64 bit rather than 32 bit (and there are 16 of them rather than 8). (You also get 16 (128 bit) SSE registers instead of the usual 8.) A decent compiler will therefore often be able to generate more efficient code (less register spill) in x86-64 code compared to old skool 32 bit code.