Chapter 13: The MMX Instruction Set

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13.1 Chapter Overview

While working on the Pentium and Pentium Pro processors, Intel was also developing an instruction set architecture extension for multimedia applications. By studying several existing multimedia applications, by developing lots of multimedia related algorithms, and through simulation, Intel developed 57 instructions that would greatly accelerate the execution of multimedia applications. The end result was the multimedia extensions to the Pentium processor that Intel calls the MMX Technology Instructions.

Prior to the invention of the MMX enhancements, good-quality multimedia systems required separate digital signal processors and special electronics to handle much of the multimedia workload.^[1] The introduction of the MMX instruction set allowed later Pentium processors to handle these multimedia tasks without these expensive digital signal processors (DSPs), thus lowering the cost of multimedia systems. So later Pentium, Pentium II, Pentium III, and Pentium IV processors all have the MMX instruction set. Earlier Pentiums (and CPUs prior to the Pentium) and the Pentium Pro do not have these instructions available. Because the instruction set has been available for quite some time, you can probably use the MMX instructions without worrying about your software failing on many machines.

In this chapter we will discuss the MMX Technology Instructions and how to use them in your assembly language programs. The use of MMX instructions, while not completely limited to assembly language, is one area where assembly language truly shines because most high level languages do not make good use of MMX instructions except in library routines. Therefore, writing fast code that uses MMX instructions is mainly the domain of the assembly language programmer. Hence, it's a good idea to learn these instructions if you're going to write much assembly code.

^[1]A good example was the Apple Quadra 660AV and 840AV computer systems; they were built around the Motorola 68040 processor rather than a Pentium, but the 68040 was no more capable of handling multimedia applications than the Pentium. However, an on-board DSP (digital signal processor) CPU allowed the Quadras to easily handle audio applications that the 68040 could not.