chapter1-AppendixA-m2-ziavras

mode field sizes on average instr size affects

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Unformatted text preview: on instr. size • Opcode field: operation code field fi • How to encode addressing modes with operations depends on: – Range of modes – Degree of independence between opcodes & modes • Some older computers: 1-5 operands with 10 addressing modes per operand Address specifier often needed for each operand needed for each operand • Load-store computers: only 1 memory operand and only 1-2 addressing modes addressing mode encoded as part of opcode Some material is adapted from D. Culler & D. Patterson (UCB) S. Ziavras Instruction Set Encoding (2) Competing forces in instr. set encoding • Desire to have many regs. & modes (flexibility) to have many regs modes (flexibility) • Impact of reg. & mode field sizes on average instr. size (affects average program size) instr. size (affects average program size) • Desire to have instrs. encoded into lengths easy to handle in pipelining – Many desktop & server architectures use fixed-length instr. for implementation benefits ( increases average code size) average code size) Some material...
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