Von Neumann アーキテクチャと CPU

The report every computer still follows

• In June 1945 John von Neumann circulated a typed draft describing a machine that had not yet been built: the EDVAC.
• Its idea was simple and new. Keep the program in the same memory as the data, and have one processor fetch and carry out the instructions one at a time.
• The draft was never finished, but the design was copied everywhere. Your phone, your laptop and the school server are all built on it.
• This lesson takes the model apart: the memory, the processor's parts, its registers and the buses between them.

The Von Neumann model

• The Von Neumann architecture 冯·诺依曼体系结构 has a single memory, the Immediate Access Store 立即存取存储器 (IAS), holding both program instructions and data. That is the stored program 存储程序 concept.
• The processor 处理器 fetches each instruction from memory and executes it, one at a time, in address order unless a branch instruction changes the flow.
• Instructions, data and addresses all travel between processor and memory along shared buses 总线.

One memory, one processor, three buses

Why the stored program matters

• Change the program in memory and the same hardware does a different job. No rewiring, no new machine.
• A program is just data until it is fetched, so it can be loaded from a disk, sent over a network or written by another program.
• The exam wording: instructions and data are stored in the same memory, and instructions are fetched one at a time and executed by the processor.

ALU, control unit and clock

• The ALU 算术逻辑单元 (Arithmetic and Logic Unit) does the arithmetic (add, subtract) and the logic (AND, OR, comparisons), taking operands from registers and putting the result back in one.
• The control unit 控制单元 (CU) decodes each instruction and sends the control signals that carry it out: opening data paths, telling the ALU what to do, ordering memory reads and writes.
• The system clock 系统时钟 sends a steady stream of pulses. The control unit moves the cycle on one step per pulse, so every part changes state together; the clock speed 时钟频率, in GHz, is the number of pulses a second.

The whole processor is one small chip

The registers

• Registers 寄存器 are tiny, very fast stores inside the processor. General-purpose registers 通用寄存器 hold the programmer's temporary values. Special purpose registers 专用寄存器 each have one fixed job in the cycle.

Registers, control unit and ALU, linked by the buses

Worked example: "describe the role of each register"

The status register's flags

• Each flag 标志 is one bit, set or cleared by the ALU after an operation: the zero flag when a result or comparison gives zero, the carry flag when an addition overflowed the register, the negative flag when a result is negative, the overflow flag when a signed result does not fit.
• A conditional jump reads a flag to decide whether to branch. That is how IF and loops reach the hardware.
• An overflow flag can also raise an interrupt, which the next lesson handles.

The three buses

• The address bus 地址总线 carries the address the processor wants. It is one-way, processor to memory.
• The data bus 数据总线 carries the data or instruction itself. It is two-way, because reads and writes go in opposite directions.
• The control bus 控制总线 carries control signals such as read, write, clock and interrupt. It is two-way.

The three system buses connecting processor, memory and input/output

Worked example: how wide is wide enough?

Register transfer notation

• Movements between registers and memory are written in register transfer 寄存器传送 notation: MAR ← [PC] means "copy the contents of the PC into the MAR".
• Square brackets mean "the contents of", and [[MAR]] means "the contents of the location whose address is in the MAR".
• The whole fetch-execute cycle is written this way in the next lesson; the notation is the same as the pseudocode assignment arrow.

Marks that slip away

You've got it