Proper noun
x87
(computing) An extension to the x86 CPU instruction set architecture, providing direct hardware support for advanced floating-point arithmetic operations (rather than requiring them to be emulated in software).
However, the new XMM register-file allowed SSE SIMD-operations to be freely mixed with either MMX or x87 FPU ops. Source: Internet
Each 64-bit MMX register corresponds to the mantissa part of an 80-bit x87 register. Source: Internet
The upper 16 bits of the x87 registers thus go unused in MMX, and these bits are all set to ones, making them NaNs or infinities in the floating point representation. Source: Internet
MMX could not be used simultaneously with the x87 FPU instructions because the registers were reused (to allow for fast context switches). Source: Internet
The first addition allowed offloading of basic floating-point operations from the x87 stack and the second made MMX almost obsolete and allowed the instructions to be realistically targeted by conventional compilers. Source: Internet