The CPU and the fetch-execute cycle
Introduced| English | Meaning |
|---|---|
| CPU/ˌsiː piː ˈjuː/ | ALU (maths/logic) + CU (control signals) + registers, linked by address/data/control buses. |
| instructions/ɪnˈstrʌkʃnz/ | Commands specifying operations for a processor or program to perform. |
| ALU/ˌeɪ el ˈjuː/ | The part of a processor that performs arithmetic and logical operations. |
| control unit/kənˈtrəʊl ˈjuːnɪt/ | The part of a processor that coordinates instruction execution and directs other components using control signals. |
| registers/ˈredʒɪstəz/ | A small, fast storage location within a processor used to hold data, addresses or instructions temporarily. |
| buses/ˈbʌsɪz/ | Communication pathways that transfer data, addresses or control signals between computer components. |
| cache/kæʃ/ | Fast storage holding copies of frequently needed data or instructions to reduce access time. |
| embedded system/emˈbedɪd ˈsɪstəm/ | A small computer built into a device for one fixed job (washing machine, microwave). |
| clock speed/klɒk spiːd/ | The frequency of a processor's timing signal, measured in hertz; it does not alone determine overall performance. |
The central processing unit
- The CPU 中央处理器 processes data and carries out instructions 指令, repeating a cycle millions of times a second.
- Inside are the ALU 算术逻辑单元, control unit 控制单元 and registers 寄存器, linked by buses 总线.
- It runs every program through the fetch–execute cycle.
Parts of the CPU
- ALU — does calculations and logical comparisons.
- Control unit (CU) — sends control signals to manage all the parts.
- Registers — tiny, very fast stores holding one value each.
- Three buses link them to memory: address (one way), data (both ways), control.
A modern CPU is a single chip holding the control unit, ALU and registers
The ALU in a CPU:
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The arithmetic logic unit does the maths and logic; the control unit manages the parts.
Registers and the fetch–execute cycle
- Special registers: PC (address of the next instruction), MAR (address to access), MDR (data/instruction fetched), CIR (instruction being run), ACC (the ALU's result).
- The cycle repeats three stages: Fetch (copy the instruction into the CPU, then PC + 1) → Decode (the CU works out what it means) → Execute (carry it out).
The fetch–decode–execute cycle that the CPU repeats for every instruction
The fetch-execute cycle
Tap round the loop the CPU repeats billions of times a second — fetch the instruction, decode what it means, execute it, then do it all again.
Match each CPU part to what it does.
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The ALU computes, the control unit directs, the PC tracks the next instruction, and cache keeps hot data close.
Put the stages of the cycle in order.
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Fetch the instruction, decode what it means, then execute it — repeated continuously.
Cache 高速缓存, performance, embedded systems 嵌入式系统
- Cache is a small amount of very fast memory near the CPU, holding often-used data — so the CPU waits less.
- Performance depends on the number of cores, the cache size, and the clock speed 时钟速度 (cycles per second).
- An embedded system is a small computer built into a device for one fixed job (washing machine, microwave).
Cache and main memory (RAM) sit close to the CPU to feed it data quickly
Which change would improve CPU performance?
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More cores, a larger cache and a higher clock speed all improve performance.
Cache is small, very fast memory near the CPU that holds often-used data, so the CPU waits less than it would reading from main memory.
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A bigger cache, more cores and a higher clock speed all raise performance — cache helps by cutting the wait for data.
You've got it
- CPU = ALU (maths/logic) + CU (control signals) + registers, linked by address/data/control buses
- key registers: PC (next instruction), MAR/MDR (memory), CIR (current), ACC (result)
- the cycle: fetch → decode → execute
- performance: cores, cache size, clock speed; cache = fast memory near the CPU