RISC (Reduced Instruction Set Computers)
Definition
RISC processors utilise a small, highly optimised set of instructions. Each instruction is designed to be executed within a single clock cycle, resulting in efficient, predictable performance.
assembly
// Multiplying two numbers in RISC:
LDA R1, X ; Load the value of X into register R1
LDA R2, Y ; Load the value of Y into register R2
MULT R1, R2 ; Multiply the values in R1 and R2
STO R1, X ; Store the result back in XKey Point
Key Characteristics of RISC:
• Simple instruction set, each executing in a single clock cycle
• Emphasis on software to perform complex tasks
• Pipelining is straightforward as each instruction is uniform in execution time
• Compiler must perform more work, translating high-level code into numerous simple instructions
• Requires more RAM to store longer sets of instructions
• Simple instruction set, each executing in a single clock cycle
• Emphasis on software to perform complex tasks
• Pipelining is straightforward as each instruction is uniform in execution time
• Compiler must perform more work, translating high-level code into numerous simple instructions
• Requires more RAM to store longer sets of instructions
CISC (Complex Instruction Set Computers)
Definition
CISC processors have a larger and more complex set of instructions, some of which can perform multi-step operations in a single command. This design reduces the number of instructions required for a task but increases the complexity of each instruction.
assembly
// Multiplying two numbers in CISC:
MULT A, B ; Multiply the values of A and B
; One instruction does what takes four in RISCKey Point
Key Characteristics of CISC:
• Larger instruction set with more complex commands
• Emphasis on reducing the number of instructions per program
• Individual instructions may take multiple clock cycles
• Compiler's job is easier with fewer instructions needed
• Requires less RAM because instructions are shorter and more complex
• Many specialised instructions, though some may remain unused
• Larger instruction set with more complex commands
• Emphasis on reducing the number of instructions per program
• Individual instructions may take multiple clock cycles
• Compiler's job is easier with fewer instructions needed
• Requires less RAM because instructions are shorter and more complex
• Many specialised instructions, though some may remain unused
RISC vs CISC Comparison
Key Point
Instruction Set:
• RISC: Small, simple instructions
• CISC: Large, complex instructions
Clock Cycles:
• RISC: 1 per instruction
• CISC: Multiple per complex instruction
Compiler Work:
• RISC: More work required
• CISC: Less work required
RAM Usage:
• RISC: More RAM needed
• CISC: Less RAM needed
Pipelining:
• RISC: Straightforward
• CISC: More complex
Emphasis:
• RISC: Software complexity
• CISC: Hardware complexity
• RISC: Small, simple instructions
• CISC: Large, complex instructions
Clock Cycles:
• RISC: 1 per instruction
• CISC: Multiple per complex instruction
Compiler Work:
• RISC: More work required
• CISC: Less work required
RAM Usage:
• RISC: More RAM needed
• CISC: Less RAM needed
Pipelining:
• RISC: Straightforward
• CISC: More complex
Emphasis:
• RISC: Software complexity
• CISC: Hardware complexity
Exam Tip
Exam Focus: Be prepared to compare RISC and CISC architectures. Key points: instruction complexity vs quantity, compiler vs hardware emphasis, pipelining efficiency.
Uses of RISC and CISC Processors
The differences in design decide where each type of processor is used.
| Feature | RISC | CISC |
|---|---|---|
| Circuitry | Simpler, with fewer transistors | More complex, with more transistors |
| Power and heat | Uses less power and produces less heat, so it needs little or no cooling | Uses more power and produces more heat, so it needs cooling |
| Cost | Cheaper to design and manufacture | More expensive to design and manufacture |
| Addressing modes | Fewer | More |
| Typical use | Smartphones, tablets and embedded systems | Desktop computers, laptops and servers |
Key Point
Why RISC suits mobile and embedded devices:
• It uses less power, so the device can have a smaller battery or run for longer on one charge
• It produces less heat, so no fan or heat sink is needed and the device can be small and light
• Its simpler circuitry makes the processor cheaper to produce
Why CISC is used in desktops and servers:
• Mains power and space for cooling mean that power use and heat matter less
• A program needs fewer instructions, so it takes up less RAM
• Existing desktop software is written for it and runs without being rewritten
• It uses less power, so the device can have a smaller battery or run for longer on one charge
• It produces less heat, so no fan or heat sink is needed and the device can be small and light
• Its simpler circuitry makes the processor cheaper to produce
Why CISC is used in desktops and servers:
• Mains power and space for cooling mean that power use and heat matter less
• A program needs fewer instructions, so it takes up less RAM
• Existing desktop software is written for it and runs without being rewritten
Exam Tip
Software written for a CISC processor cannot run directly on a RISC processor, because the instruction sets are different. In a scenario question, link each point to the device: a RISC processor in a phone gives a longer battery life, which is more than saying it uses less power.