Differences in ISA

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 CA-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?

• RISP
• many instruction
• one cycle per instruction
• High cycle time

• CISP
• few instruction
• Multiple cycle per instruction
• Low 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