RFRISC vs CISC: Key Architectural Differences Explained
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RISC stands for Reduced Instruction Set Computer architecture, where emphasis is given to software design. Examples of RISC architectures include MIPS, SUN Sparc, DEC Alpha, and IBM801.
CISC stands for Complex Instruction Set Computer architecture, where emphasis is given to hardware design. Examples of CISC architectures include VAX, Intel x86, and IBM 360/IBM 370.
Figure depicts microprocessors of both RISC and CISC types.
RISC processors typically result in a larger program size, while CISC processors result in a smaller program size. The compiler for RISC architectures is generally more complex in design, whereas the compiler for CISC architectures is simpler. RISC uses single-word instructions, while CISC uses variable-length instructions.
RISC supports load/store architecture, while CISC supports memory operands instructions. RISC does not support indirect addressing mode.
Understanding the fundamental differences between RISC and CISC helps in making informed decisions about processor selection and optimization for various computing needs.
Let’s delve into a comparison between RISC and CISC to highlight the differences between these processor types:
| Parameters | RISC | CISC |
|---|---|---|
| Full form | Reduced Instruction Set Computer | Complex Instruction Set Computer |
| About Instruction set | Compact, uniform instructions, facilitate pipelining, highly optimized set of instructions | Many addressing modes, long instructions, complex set of instructions |
| Instruction length | Fixed | Variable |
| Execution time | Each instruction typically takes one clock cycle | Instructions can take multiple clock cycles |
| Pipelining | Designed for easy pipelining | More challenging to pipeline due to instruction complexity |
| Memory access | Load/Store architecture; separate load and store instructions | Instructions can perform multiple operations, including memory access |
| Decoding | Simple | Complex |
| Registers | Larger | Fewer |
| Power consumption | Generally lower | Generally higher |
| About Source code | More lines of code, large memory footprint | High code density |
| About Optimization | Allow effective compiler optimization | Often require manual optimization of assembly code for embedded systems |
| Hardware design complexity | Simpler | More complex |
| Software programming | Requires more instructions to perform complex tasks | Can perform complex tasks with single instructions |
| Compiler design | More responsibility on compiler to optimize instruction usage | Less responsibility on compiler as instructions are more capable |
| Examples | ARM Cortex-A, MIPS, RISC-V, IBM power systems, SPARC | Intel x86, AMD x86, IBM System/360 etc. |
Conclusion
The RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) architectures represent two different approaches to CPU design, each with its own strengths and weaknesses.
RISC is preferred when efficiency, simplicity, and power consumption are critical factors.
CISC is chosen for its ability to handle complex instructions and reduce the number of instructions per program, which can be beneficial for certain computing tasks.