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SIMD (Single Instruction Multiple Data) Processing - Advantages of SIMD, Wireless MMX Technology

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Definition: SIMD processing, in which single instruction is applied on multiple data, is suitable for multimedia processing, and therefore it is implemented in contemporary processors.

Single instruction multiple data (SIMD), as the name suggests, takes an operation specified in one instruction and applies it to more than one set of data elements at the same time. For example, in a traditional scalar microprocessor, an add operation would add together a single pair of operands and produce a single result. In SIMD processing, a number of independent operand pairs are added together to produce the same number of independent sums. Figure 1 illustrates traditional and SIMD processing.

The number of parallel data elements processed at the same time is dependent on the size of the elements and the capacity of the data processing resources available. Usually the capacity of the data processing resources is fixed for a given architecture.

Advantages of SIMD

The main advantage of SIMD is that processing multiple data elements at the same time, with a single instruction, can dramatically improve performance. For example, processing 12 data items could take 12 instructions for scalar processing, but would require only three instructions if four data elements are processed per instruction using Page 819  SIMD. While the exact increase in code speed that you observe depends on many factors, you can achieve a dramatic performance boost if SIMD techniques can be utilized. Not everything is suitable for SIMD processing, and not all parts of an application need to be SIMD accelerated to realize significant improvements.

Wireless MMX Technology

The Wireless MMX unit is an example of a SIMD coprocessor. It is a 64-bit architecture that is is an extension of the XScale microarchitecture programming model. Wireless MMX technology defines three packed data types (8-bit byte, 16-bit half word and 32-bit word) and the 64-bit double word. The elements in these packed data types may be represented as signed or unsigned fixed point integers Wireless MMX technology comprises five key functional units to implement the programmers model. Figure 2 shows the organization of the functional units within the coprocessor.

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