3-3  VLIW Microprocessors

When Transmeta Corp., revealed its new Crusoe family of processors last month, experts weren't surprised to learn that the chips are based on Very Long Instruction Word (VLIW) technology. VLIW has become the prevailing philosophy of microprocessor design, eclipsing older approaches such as RISC and complex instruction set computing (CISC).[1]

All microprocessor designs seek better performance within the limitations of their contemporary technology. In the 70s of 20th century, for example, memory was measured in kilobytes and was very expensive. CISC was the dominant approach because it conserved memory.

In the CISC architecture, there can be hundreds of program instructions—simple commands that tell the system to add numbers, store values and display results. If all instructions were the same length, the simple ones would waste memory. Simple instructions require as little as 8 bits of storage space, while the most complex consume 120 bits.

Variable-length instructions are more difficult for a chip to process, though, and the longer CISC instructions are especially complex. Nonetheless, to maintain software compatibility, modem chips such as Intel's Pentium III and Advanced Micro Devices Inc. 's Athlon must still work with all troublesome CISC instructions that were designed in the 1980s, even though their original advantage—memory conservation—isn't as important.

In the 1980s, RAM chips got bigger and bigger in capacity while their prices dropped. The emphasis in CPU design shifted to raw performance, and RISC became the new philosophy.

RISC chips use a rather small number of relatively simple, fixed-length instructions, always 32 bits long. Although this wastes some memory by making programs bigger, the instructions are easier and faster to execute.

The simplicity of RISC also makes it easier to design superscalar processors—chips that can execute more than one instruction at a time. This is called instruction-level parallelism. Almost all modem RISC and CISC processors are superscalar. But achieving this capability introduced significant new levels of design complexity.

VLIW is the latest way to simplify processors. VLIW chips don't need most of the complex control circuitry that superscalar chips must use to coordinate parallel execution at runtime. Instead, VLIW chips shift more of that burden onto compilers.

Also, VLIW chips combine two or more instructions into a single bundle or packet. The compiler prearranges the bundles so the VLIW chip can quickly execute the instructions in parallel, freeing the microprocessor from having to perform the complex and continual runtime analysis that superscalar RISC and CISC chips must do.

VLIW chips can cost less, bum less power and achieve significantly higher performance than comparable RISC and CISC chips. But there are always trade-offs. One is code expansion-programs grow larger, requiring more memory. Far more important, though, is that compilers must get smarter. A poor VLIW compiler will have a much greater negative impact on performance than would a poor RISC or CISC compiler.

VLIW isn't a magic bullet, but it's the new wave in microprocessor design. Within a few years, it's certain that at least some of your software will be running on VLIW chips.

WORDS AND PHRASES

eclipse    侵蚀，黯然失色

prevail   获胜，流行

trade-off 折衷，一分为二

NOTES

[1] VLIW has become the prevailing philosophy of microprocessor design, eclipsing older approaches such as RISC and complex instruction set computing (CISC).

特长指令字已成为微处理器最为流行的设计原则，它使较为陈旧的RISC和复杂指令集计算CISC黯然失色。