Cambridge A-Level Computer Science (9618) · lesson 12 of 15 · 30 min
Instruction Set Architectures
An instruction set architecture (ISA) defines the set of instructions a CPU can execute, the registers available, memory addressing modes, and data types. The two dominant philosophies are CISC and RISC.
CISC vs RISC
Why ARM Dominates Mobile
ARM processors use a RISC design. Simple, fixed-length instructions mean:
- Efficient pipelining → high performance per watt.
- Simpler hardware → smaller die size → lower cost and heat.
- Ideal for battery-powered devices (phones, tablets, IoT).
Modern x86 CPUs internally translate CISC instructions into RISC-like micro-operations (μops) before execution — blurring the distinction.
Which ISA philosophy uses simple, fixed-length instructions that mostly execute in one cycle?
- CISC
- RISC
- Both
- Neither
Answer: RISC
RISC uses simple, uniform instructions designed to execute in one clock cycle.
RISC processors are preferred in mobile devices because:
- They have more instructions
- They are more power-efficient
- They support more memory
- They are cheaper to manufacture
- Answer: 1
RISC’s simple instructions allow efficient pipelining and lower power consumption — critical for battery life. In RISC architecture, memory access is restricted to:
- Any instruction
- Only arithmetic instructions
- Load and store instructions only
- Jump instructions only
- Answer: 2
RISC uses a load/store architecture — only LOAD and STORE instructions access memory; all other operations work on registers.
Previous lesson: Processor architecture: Harvard vs von Neumann
Back to Cambridge A-Level Computer Science (9618)
Next lesson: Pipelining and parallel processing