Page "X86" Paragraph 17

SSE discarded all legacy connections to the FPU stack.

This also meant that this instruction set discarded all legacy connections to previous generations of SIMD instruction sets like MMX.

But it freed the designers up, allowing them to use larger registers, not limited by the size of the FPU registers.

The designers created eight 128-bit registers, named XMM0 through XMM7.

( Note: in AMD64, the number of SSE XMM registers has been increased from 8 to 16.

) However, the downside was that operating systems had to have an awareness of this new set of instructions in order to be able to save their register states.

So Intel created a slightly modified version of Protected mode, called Enhanced mode which enables the usage of SSE instructions, whereas they stay disabled in regular Protected mode.

An OS that is aware of SSE will activate Enhanced mode, whereas an unaware OS will only enter into traditional Protected mode.