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Unformatted text preview: c A.H.Dixon CMPT 250 : Week 5 (Oct 4 - 8) 24 11 INSTRUCTION DESIGN With an understanding of the parts of an instruction and their purpose, the design of instructions is influenced by several major factors: 1. Size of the instruction set. Instructions set size depends on a design decision as to whether a reduced in- struction set computer (RISC) or a complex instruction set computer (CISC) is to be adopted. RISC Philosophy : A small number of general purpose instructions will define the instruction set: A small number of instructions means a small opcode and shortens the length of a (machine) instruction. A small number of instructions leads to simpler CPU architectures. Simpler CPU architectures are easier and more economical to fabricate. CISC Philosophy : A large number of instructions allows the inclusion of special purpose instructions: Performance gains can be obtained by customizing the datapath to ac- commodate complex special purpose instructions,. Variations on instructions provide the programmer with more choice in the implementation of an algorithm. Less need for subprograms to be defined as such functions can be imple- mented as instructions. Generally 64 or fewer instructions constitutes a RISC architecture while 256 or more instructions is considered a CISC architecture. The decision as to which approach to take depends on the trade-offs between performance priorities, pro- gramming convenience, and economic limitations. The likelihood that a particular instruction will be used is also a factor in its inclusion in a given instruction set. In both cases, however, the most commonly used instructions should be designed to also be the most efficiently implemented. 2. Definition of Instruction Formats. The design of instruction formations determines how the instructions will be im- plemented as binary sequences. Instruction sets should require as few different instruction formats as are reasonable. While a minimum number may be desirable, practical imple- mentation considerations may require that additional formats be adopted. The roles of similar fields in different formats should be located in the same positions within each format. Similar features of two instructions should be implemented in similar ways. Instruction format design tasks: c A.H.Dixon CMPT 250 : Week 5 (Oct 4 - 8) 25 (a) Assign opcodes to all instructions and determine their size (i.e., number of bits required) (b) Determine the number and size of each operand to be encoded. (c) Classify instructions, groups them into sets having the same or similar operand requirements....
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This note was uploaded on 03/16/2012 for the course ENSC 802 taught by Professor Ivanbaijic during the Spring '12 term at Simon Fraser.
- Spring '12