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MMX (instruction set)

MMX is a single instruction, multiple data (SIMD) instruction set designed by Intel, introduced in 1997 in their Pentium line of microprocessors, designated as "Pentium with MMX Technology". It developed out of a similar unit first introduced on the Intel i860. It has been supported on most subsequent IA-32 processors by Intel and other vendors.

Intel's competitor AMD enhanced Intel's MMX with the 3DNow! instruction set to work with floating-point numbers. Intel would follow AMD's lead on floating-point math and created the SSE extension two years later.

Naming

MMX is officially a meaningless initialism trademarked by Intel; unofficially, the initials have been variously explained as standing for MultiMedia eXtension, Multiple Math eXtension, or Matrix Math eXtension.

AMD, during one of its numerous court battles with Intel, produced marketing material from Intel indicating that MMX stood for "Matrix Math Extensions". The idea that it stands for nothing is an Intel corporate position meant to suggest that it is of trademarked status and cannot be used by AMD or other x86 clone manufacturers in their own marketing material.

Technical details

MMX added eight new registers to the architecture, known as MM0 through MM7 (henceforth referred to as MMn). In reality, these new "registers" were just aliases for the existing x87 FPU stack registers. Hence, anything that was done to the floating point stack would also affect the MMX registers. Unlike the FP stack, these MMn registers were fixed, not relative, and therefore they were randomly accessible.

Each of the MMn registers is a 64-bit integer. However, one of the main concepts of the MMX instruction set is the concept of packed data types, which means instead of using the whole register for a single 64-bit integer (quadword), two 32-bit integers (doubleword), four 16-bit integers (word) or eight 8-bit integers (byte) may be used.

To simplify the design and to avoid modifying the operating system to preserve additional state through context switches, MMX re-uses the existing eight IA-32 FPU registers. This made it difficult to work with floating point and SIMD data at the same time. To maximize performance, programmers must use the processor exclusively in one mode or the other, deferring the relatively slow switch between them as long as possible.

Also because the MMX's 64-bit MMn registers are aliased to the FPU stack, and each of the stack registers is 80 bits wide, the upper 16 bits of the stack registers go unused in MMX, and these bits are set to all ones, which makes it look like NaNs or infinities in the floating point view. This makes it easier to tell whether the work is on floating point data or MMX data.

Another problem for MMX is that it only provides integer operations. Each of the eight 64-bit MMX vector registers, aliased on the eight existing floating point registers, could represent two 32-bit integers, four 16-bit short integers, or eight 8-bit bytes. When originally developed in the i860, the use of vectored-integer math made sense (both 2D and 3D setup required it), but as the systems moved to using graphics cards that did this, MMX fell out of favor and vectored-floating point became much more important. On the other hand, its new arithmetic operations did include saturation arithmetic operations, which could significantly speed up some digital signal processing applications.

Successor

Intel addressed the shortcomings of the MMX technology through SSE, a greatly expanded set of SIMD instructions with 32-bit floating point support and an additional set of 128-bit vector registers that made it easy to perform SIMD and FPU operations at the same time. SSE was in turn expanded with SSE2, which also extended MMX instructions so they can operate on 128-bit XMM registers (later SSE extensions would still support operating integer data on MMX registers because the new SSE registers require OS support, until SSE4, which ended this support) and recently with SSE4.2, introduced in the Intel Core microarchitecture. Support for any of these later instruction sets implies support for MMX.

MMX in embedded applications

Intel's XScale processors starting with PXA270 include an extension to the ARM core called iwMMXt whose functions are similar to those of the IA-32 MMX extension. IwMMXt stands for "Intel Wireless MMX Technology". It provides arithmetic and logic operations on 64-bit integer numbers (the software may choose to instead perform two 32-bit, four 16-bit or eight 8-bit operations in a single instruction). The extension contains 16 data registers of 64-bits and eight control registers of 32-bits. All registers are accessed through standard ARM architecture coprocessor mapping mechanism. IwMMXt occupies coprocessors 0 and 1 space, and its opcodes coincide with the opcodes of an earlier floating-point extension, FPA.