An Overview of Intel's MMX Technology

Commercially introduced in January 1997, the MMX technology is an extension of the Intel architecture that uses a single-instruction, multiple-data execution model that allows several data elements to be processed simultaneously. Applications that benefit from the MMX technology are those that do many parallelizable computations using small integer numbers. Examples of these kinds of applications are 2-D/3-D graphics, image processing, virtual reality, audio synthesis and data compression.

If your Linux system has a Pentium II or a Pentium with MMX technology, you can build programs that take advantage of the MMX instruction set using gcc and a bit of assembly language. In this article, I will briefly introduce the main features of the MMX technology, explain how to detect whether an x86 microprocessor has built-in MMX capabilities and show how to program a simple image processing application.

The assembly language code presented here uses NASM, the Netwide Assembler. NASM employs the standard Intel syntax instead of the AT&T syntax used on many popular UNIX assemblers, such as GAS.

Figure 1. MMX Register Set

The MMX technology extends the Intel architecture by adding eight 64-bit registers and 57 instructions. The new registers are named MM0 to MM7 (see Figure 1). Depending on which instructions we use, each register may be interpreted as one 64-bit quadword, two packed 32-bit double words, four packed 16-bit words, or eight packed 8-bit bytes (see Figure 2).

Figure 2. MMX Data Types

The MMX instruction set comprises several categories of instructions, including those for arithmetic, logical, comparison, conversion and data transfer operations.

The syntax for MMX instructions is similar to other x86 instructions:

OP Destination, Source

This line is interpreted as:

Destination = Destination OP Source

Before running a program that uses MMX instructions, it is important to make sure your microprocessor actually has MMX support. Your Linux system should be an Intel x86 or compatible microprocessor (386, 486, Pentium, Pentium Pro, Pentium II, or any of the Cyrix or AMD clones). This is easily checked by executing the uname -m command. This command should return i386, i486, i586 or i686. If it does not, your Linux system runs on a non-x86 architecture.

In order to determine if your CPU supports MMX technology, use the assembly language CPUID instruction. This instruction reveals important processor information, such as its vendor, family, model and cache information. Unfortunately, the CPUID instruction is present only on some late 80486 processors and above. So, how do you know if CPUID is available on your system? Intel documents the following trick: if your program can modify bit 21 of the EFLAGS register, then the CPUID instruction is available; otherwise, you are working with an aged CPU. See Listing 1 (lines 12-29) to learn how this can be done.

Listing 1.

Next, request CPU feature information by putting a value of 1 in the EAX register and executing the instruction. The resulting value is returned in bit 23 of the EDX register. If this bit is on, the processor supports the MMX instruction set; otherwise, it does not. Listing 1 (lines 43-50) shows how to do this.

Programs should contain two versions of the same routine: one using MMX technology and one using regular scalar code. At runtime, the program can decide which routine it should actually call.

If MMX instructions are executed in a system that does not support them, the CPU will raise an “invalid opcode exception” (interrupt vector number 6) which is trapped by the Linux kernel. The Linux kernel in turn sends an “illegal instruction signal” (code number 4) to the offending process. By default, this action terminates the program and generates a core file.

Figure 3. Original Gray-Scale Image