Programming Applications for the AltiVec Architecture

For years, DSPs held the signalprocessing performance edge. Fast RISCs could not compete against the DSP's CISC architectures. Today, that's no longer true. Developers can get DSP performance with RISC programmability on a standard PowerPC architecture. Motorola's AltiVec technology has turned the PowerPC RISC into a signalprocessing powerhouse that can take advantage of existing PowerPC software and tools.

This is not just a slam-dunk in the DSP vs. RISC wars. For while AltiVec tilts the scales decidedly in favor of RISC-based computing for embedded real-time signal and image processing, the traditional DSP community is not standing still. Their continuing adoption of architectural elements from general-purpose processors may ultimately lead them to products that are as easy to use as RISC processors such as the next-generation PowerPC with AltiVec. The open question is, within the same time frame, how much further will the PowerPC and other RISC architectures advance their own ease of use?

Today's RISC processors, most notably the PowerPC family, have achieved clock speeds that allow their raw performance to surpass DSP processors for typical vector operations. In the future, the fourth-generation (G4) PowerPC microprocessor may offer up to a fourfold increase over the current generation of PowerPC 750 microprocessors.

Much of this performance boost comes from Motorola's new AltiVec technology, which adds DSP-like capability to the PowerPC architecture. AltiVec technology expands the chip's processing capabilities through the addition of a 128-bit vector execution unit that operates concurrently with existing integer and floating-point units. This new engine permits highly parallel operations, allowing for the simultaneous execution of up to 16 operations in a single clock cycle.

AltiVec works with a set of SIMD instructions that dramatically increases the computational efficiency of a PowerPC processor. Programmers continue to enjoy all the software advantages of the friendly PowerPC core; extra transistors dedicated to AltiVec are available to handle DSP tasks, at the cost of a little extra programming effort.

As a RISC processor, a PowerPC processor with AltiVec technology also contains a PowerPC memory management unit (MMU). The MMU lets the processor work with virtual addresses, providing memory protection for embedded real-time applications. Without such protection, a simple memory access violation can cause a complex set of system-level symptoms that can be hard to diagnose.

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