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Differences in ISA

Differences in ISA. Instruction length Fixed versus variable, 2 byte to 57 byte Number of different instructions (opcodes) Addressing modes Number and Type of operands Stack, Accumulator, Multiple Register, Memory Size of operands (related to length). Number of Instructions.

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Differences in ISA

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  1. Differences in ISA • Instruction length • Fixed versus variable, 2 byte to 57 byte • Number of different instructions (opcodes) • Addressing modes • Number and Type of operands • Stack, Accumulator, Multiple Register, Memory • Size of operands (related to length)

  2. Number of Instructions • RISC: Reduced instruction set computer • Fewer instructions • Fixed length • Short opcode • CISC: Complex instruction set computer • More instruction • Variable length • Longer opcode

  3. Example: Subtraction C=A-B RISC: LOAD R1, A LOAD R2, B NOT R2, R2 ADD R2, R2, 1 ADD R2, R2, R1 STORE C,R2 CISC: SUBMEM C, A, B

  4. Which is better? • Better in what terms? • Speed • Cost of production • Cost of development • Flexibility for improvement

  5. Better in terms of speed? • CISP: • few instruction • Multiple cycle per instruction • Low cycle time • RISP: • many instruction • one cycle per instruction • High cycle time No clear answer!

  6. Examples for CISC and RISC • CISC • Intel Pentium • AMD • IBM 390 • RISC • HP PA • Motorola, IBM,Apple: PowerPc • Sun Ultra: SPARC

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