Processors: RISC vs CISC
1. The Two Philosophies
Processor design is a trade-off between Hardware Complexity and Software Simplicity.
RISC
Reduced Instruction Set Computer
Uses a small set of simple, fixed-length instructions. Each instruction takes exactly one clock cycle.
Examples: ARM (iPhone, Android, M-series Macs).
CISC
Complex Instruction Set Computer
Uses a large set of varied instructions. One complex instruction might take multiple clock cycles.
Examples: Intel x86, AMD (Desktop PCs, Laptops).
CISC is like an "All-in-One Food Processor." It has one button to "Make Pizza." It’s complicated inside, but the cook (programmer) only has to push one button.
RISC is like a set of "Sharp Knives." You have to chop the dough, slice the cheese, and heat the oven yourself. It’s simpler to build, and because the knives are so efficient, a fast cook can make the pizza just as quickly as the machine.
2. Comparison Summary
| Feature | RISC | CISC |
|---|---|---|
| Instruction Length | Fixed (e.g., 32-bit) | Variable |
| Cycles per Instruction | 1 Cycle | Multiple Cycles |
| Pipelining | Very easy to implement | Very difficult to implement |
| Energy Consumption | Low (Better for mobile) | High (Requires more cooling) |
| Compiler Work | Heavy (Must break code down) | Light (One command does a lot) |
3. Pipelining
Because RISC instructions are all the same length and take the same time, we can use Pipelining. This allows the processor to work on different stages of multiple instructions at once (Fetch, Decode, Execute simultaneously).
In a Paper 3 exam, remember: CISC aims to complete a task in as few lines of assembly code as possible, while RISC aims to complete each line of assembly as fast as possible.