The CPU and the fetch-execute cycle · ⁨CPU 与取指-执行周期⁩

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

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

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

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