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Unformatted text preview: ies to achieve the logic density required to fit the complete processor onto a single chip. RISC drawbacks RISC processors have clearly won the performance battle and should cost less to design, so is a RISC all good news? With the passage of time, two drawbacks have come to light: RISCs generally have poor code density compared with CISCs. RISCs don't execute x86 code. The second of these is hard to fix, though PC emulation software is available for many RISC platforms. It is only a problem, however, if you want to build an IBM PC compatible; for other applications it can safely be ignored. The poor code density is a consequence of the fixed-length instruction set and is rather more serious for a wide range of applications. In the absence of a cache, poor code density leads to more main memory bandwidth being used for instruction fetching, resulting in a higher memory power consumption. When the processor incorporates an on-chip cache of a particular size, poor code density results in a smaller proportion of the working set being held in the cache at any time, increasing the cache miss rate, resulting in an even greater increase in the main memory bandwidth requirement and consequent power consumption. ARM code density and Thumb The ARM processor design is based on RISC principles, but for various reasons suffers less from poor code density than most other RISCs. Its code density is still, however, not as good as some CISC processors. Where code density is of prime importance, ARM Limited has incorporated a novel mechanism, called the Thumb architecture, into some versions of the ARM processor. The Thumb instruction set is a 16-bit compressed form of the original 32-bit ARM instruction set, and employs dynamic decompression hardware in the instruction pipeline. Thumb code density is better than that achieved by most CISC processors. The Thumb architecture is described in Chapter 7. 28 An Introduction to Processor Design Beyond RISC It seems unlikely that RISC represents the last word on computer architecture, so is there any sign of another breakthrough which will render the RISC approach obsolete?...
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- Spring '09