ISA - Instruction Set Architecture 5.1 Chapter Overview...

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Page 270 Instruction Set Ar chitecture Chapter Five 5.1 Chapter Overview This chapter discusses the low-level implementation of the 80x86 instruction set. It describes how the Intel engineers decided to encode the instructions in a numeric format (suitable for storage in memory) and it dis - cusses the trade-of fs they had to make when designing the CPU. This chapter also presents a historical back - ground of the design ef fort so you can better understand the compromises they had to make. 5.2 The Importance of the Design of the Instruction Set In this chapter we will be exploring one of the most interesting and important aspects of CPU design: the design of the CPU s instruction set. The instruction set architecture (or ISA) is one of the most important design issues that a CPU designer must get right from the start. Features like caches, pipelining, superscalar implemen - tation, etc., can all be grafted on to a CPU design long after the original design is obsolete. However , it is very dif ficult to change the instructions a CPU executes once the CPU is in production and people are writing soft - ware that uses those instructions. Therefore, one must carefully choose the instructions for a CPU. Y ou might be tempted to take the "kitchen sink" approach to instruction set design 1 and include as many instructions as you can dream up in your instruction set. This approach fails for several reasons we ll discuss in the following paragraphs. Instruction set design is the epitome of compromise management. Good CPU design is the process of selecting what to throw out rather than what to leave in. It s easy enough to say "let s include everything." The hard part is deciding what to leave out once you realize you can t put everything on the chip. Nasty r eality #1 : Silicon r eal estate . The first problem with "putting it all on the chip" is that each feature requires some number of transistors on the CPU s silicon die. CPU designers work with a "silicon budget" and are given a finite number of transistors to work with. This means that there aren t enough transistors to support "putting all the features" on a CPU. The original 8086 processor , for example, had a transistor budget of less than 30,000 transistors. The Pentium III processor had a budget of over eight million transistors. These two bud - gets reflect the dif ferences in semiconductor technology in 1978 vs. 1998. Nasty r eality #2 : Cost . Although it is possible to use millions of transistors on a CPU today , the more tran - sistors you use the more expensive the CPU. Pentium IV processors, for example, cost hundreds of dollars (circa 2002). A CPU with only 30,000 transistors (also circa 2002) would cost only a few dollars. For low-cost sys - tems it may be more important to shave some features and use fewer transistors, thus lowering the CPU s cost.
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ISA - Instruction Set Architecture 5.1 Chapter Overview...

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