A Level Computer Science OCR Practice Exam

How many machine cycles are typically run per instruction on a CISC architecture?

1. One

2. Two

3. Multiple

4. None

The correct answer is: Multiple

In a CISC (Complex Instruction Set Computing) architecture, it is common for multiple machine cycles to be required to execute a single instruction. This is due to CISC's design philosophy, which allows for complex instructions that can perform multiple operations in a single instruction. Each of these complex instructions may involve various memory accesses, data manipulations, and even branching, which can necessitate separate cycles for fetching the instruction, decoding it, and performing the required operations. Because of this complexity, a single instruction could trigger several distinct machine cycles, unlike RISC (Reduced Instruction Set Computing) architectures, which typically aim to execute one instruction per cycle for efficient processing. In essence, the nature of CISC instructions often leads to a scenario where multiple machine cycles are not just common but somewhat expected to properly execute the instruction in question.