The desire to achieve more instruction level parallel

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: The desire to achieve more instruction level parallel-ism by making changing the architecture to support wider instructions (seeChapter 4).nIncreased Conditional Execution: More support for conditional execution ofoperations to support greater speculation.nBlending of general purpose and DSP architectures: Parallel efforts betweendesktop and embedded processors to add DSP support vs. extending DSP pro-cessors to make them better targets for compilers, suggesting a culture clash inthe marketplace between general purpose and DSPs.n80x86 emulation: Given the popularity of software for the 80x86 architecture,many companies are looking to see if changes to the instruction sets can signif-icantly improve performance, cost, or power when emulating the 80x86 archi-tecture.Between 1970 and 1985 many thought the primary job of the computer archi-tect was the design of instruction sets. As a result, textbooks of that era empha-size instruction set design, much as computer architecture textbooks of the 1950sand 1960s emphasized computer arithmetic. The educated architect was expectedto have strong opinions about the strengths and especially the weaknesses of thepopular computers. The importance of binary compatibility in quashing innova-tions in instruction set design was unappreciated by many researchers and text-book writers, giving the impression that many architects would get a chance todesign an instruction set.The definition of computer architecture today has been expanded to includedesign and evaluation of the full computer system—not just the definition of theinstruction set and not just the processor—and hence there are plenty of topicsfor the architect to study. (You may have guessed this the first time you lifted thisbook.) Hence, the bulk of this book is on design of computers versus instructionsets.
160Chapter 2 Instruction Set Principles and ExamplesThe many appendices may satisfy readers interested in instruction set archi-tecture: Appendix B compares seven popular load-store computers with MIPS.Appendix C describes the most widely used instruction set, the Intel 80x86, andcompares instruction counts for it with that of MIPS for several programs. Forthose interested in the historical computers, Appendix D summarizes the VAX ar-chitecture and Appendix E summarizes the IBM 360/370.One’s eyebrows should rise whenever a future architecture is developed with a stack- or register-oriented instruction set. [p. 20]Meyers [1978]The earliest computers, including the UNIVAC I, the EDSAC, and the IAS com-puters, were accumulator-based computers. The simplicity of this type of computermade it the natural choice when hardware resources were very constrained. Thefirst general-purpose register computer was the Pegasus, built by Ferranti, Ltd. in1956. The Pegasus had eight general-purpose registers, with R0 always being zero.

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