RISC vs CISC

RISC vs CISC is one of the most common COA and architecture interview questions because it checks whether you understand processor design philosophy.

What is RISC?

RISC stands for Reduced Instruction Set Computer.

Features:

• simple instructions
• fixed instruction format
• faster execution of individual instructions
• more use of registers

Examples:

• ARM
• MIPS

What is CISC?

CISC stands for Complex Instruction Set Computer.

Features:

• more complex instructions
• variable instruction length
• one instruction can perform multiple low-level operations
• more complex control logic

Example:

• x86 family

RISC vs CISC Table

| Feature | RISC | CISC | | --- | --- | --- | | Instruction style | Simple | Complex | | Instruction length | Fixed | Variable | | Execution | Faster per instruction | More work in one instruction | | Pipelining | Easier | Harder | | Hardware design | Simpler | More complex |

Interview-style answer

RISC uses a small set of simple and fast instructions, while CISC uses a larger set of more complex instructions. RISC is easier to pipeline and usually depends more on registers, while CISC tries to reduce the number of instructions per program.

Quick Revision Points

• RISC = simple and fast
• CISC = complex and feature-rich
• RISC easier for pipelining
• CISC usually has more complex instruction logic

Viva Questions

1. What is RISC?
2. What is CISC?
3. What is the difference between RISC and CISC?
4. Why is RISC easier to pipeline?
5. Give one example of RISC and one of CISC.