as supports Intel's MMX instruction set (SIMD
instructions for integer data), available on Intel's Pentium MMX
processors and Pentium II processors, AMD's K6 and K6-2 processors,
Cyrix' M2 processor, and probably others. It also supports AMD's 3DNow!
instruction set (SIMD instructions for 32-bit floating point data)
available on AMD's K6-2 processor and possibly others in the future.
as does not support Intel's floating point
SIMD, Katmai (KNI).
The eight 64-bit MMX operands, also used by 3DNow!, are called ‘%mm0’, ‘%mm1’, ... ‘%mm7’. They contain eight 8-bit integers, four 16-bit integers, two 32-bit integers, one 64-bit integer, or two 32-bit floating point values. The MMX registers cannot be used at the same time as the floating point stack.
See Intel and AMD documentation, keeping in mind that the operand order in instructions is reversed from the Intel syntax.