(MultiMedia EXtensions) A set of 57 additional instructions built into the Pentium MMX chip for improved multimedia and modem performance by performing mathematical operations on multiple sets of data at the same time (see SIMD). For example, MMX includes a multiply-add instruction that provides the capabilities of a DSP chip typically found on a sound card. All subsequent Pentium CPUs, except for the Pentium Pro, included MMX. Many multimedia enhancements to the instruction set were added later (see SSE).

MMX Borrows its Registers
The CPU has to be switched into MMX mode, which turns the first 64 bits of the eight 80-bit floating point registers into MMX registers. When 8, 16 or 32-bit integers are loaded into the MMX registers, they can be added and multiplied simultaneously. Additional integer registers were added with Intel's subsequent SSE instructions, thus simultaneous floating point and multimedia operations were no longer mutually exclusive (see SSE).