Computer Organization and Architecture2025-267 marksProgram Control, Reduced Instruction Set Computer, Pipelinin
Explain the concept of RISC architecture. Discuss its features and advantages. Compare RISC with CISC architecture.
Explain the concept of RISC architecture. Discuss its features and advantages. Compare RISC with CISC architecture.
Quick answer
RISC means Reduced Instruction Set Computer. Its idea is that most programs use a few simple instructions most of the time. So the processor is built to run these very fast, and complex jobs are made from many simple instructions.
RISC architecture
Features
- A small set of simple instructions, of fixed length, such as 32 bits.
- Each instruction takes one clock cycle, in most cases.
- Only LOAD and STORE instructions access memory. All other instructions work on registers. This is the load-store architecture.
- A large set of registers, often with overlapping register windows, which makes the procedure calls fast.
- Few and simple addressing modes.
- A hardwired control unit, with no microcode.
- A pipeline that is easy to build and keep full, as all the instructions are alike.
Advantages
- Higher speed, because of one-cycle instructions and good pipelining.
- A simple control unit, so a smaller chip, lower design time and lower cost.
- Less power use, which is the reason ARM is used in phones.
- Easier for compilers to optimize, as the instructions are regular.
Disadvantages. The programs are longer, as more instructions are needed, and they need more memory for code.
Comparison of RISC and CISC
| Point | RISC | CISC |
|---|---|---|
| Instruction set | Small, simple | Large, complex |
| Instruction length | Fixed | Variable |
| Cycles per instruction | About 1 | Many |
| Memory access | LOAD and STORE only | In many instructions |
| Addressing modes | Few | Many |
| Registers | Many | Few |
| Control unit | Hardwired | Microprogrammed |
| Pipelining | Easy and deep | Difficult |
| Code size | Larger | Smaller |
| Compiler work | More | Less |
| Examples | ARM, MIPS, SPARC | Intel x86, VAX |
In short, RISC moves the complexity from the hardware to the software, and CISC does the reverse.
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