Performance, the fetch-execute cycle and interrupts · ⁨性能、取指-执行周期与中断⁩

Why your laptop drops everything when you press a key

• A processor runs billions of cycles a second. If it had to stop and ask the keyboard "anything yet?" between instructions, it would waste most of them asking.
• Instead the keyboard sends an interrupt 中断. The processor finishes its current instruction, saves its place, deals with the key, and returns as if nothing had happened.
• That one mechanism is how a single processor appears to run your music, your browser and your typing at once.
• This lesson covers what makes a processor fast, how it connects to devices, exactly how it runs one instruction, and how an interrupt cuts in.

What affects performance

• Clock speed 时钟频率: more fetch-execute cycles a second, so more instructions a second, until heat sets the limit.
• Number of cores 核心: each core fetches and executes its own instruction at the same time, so several programs or threads run in parallel. A program must be written to use them, so doubling the cores does not double the speed.
• Bus width: a wider data bus moves more bits per transfer, so fewer transfers; a wider address bus reaches more memory.
• Cache 高速缓存: a small, fast memory next to the processor holding the instructions and data used most recently, so fewer slow trips to RAM. Word size 字长, the amount of RAM and the storage type help too.

Worked example: "explain why the new computer is faster"

Ports

• A port 端口 is a physical socket that connects a peripheral 外围设备.
• USB (Universal Serial Bus) is general-purpose: keyboards, drives, phones. It is plug-and-play: the computer detects the device, identifies it, loads its driver and can power it, with no restart.
• HDMI (High Definition Multimedia Interface) carries digital video and audio down one cable, so nothing is converted to analogue and the picture is not degraded. VGA (Video Graphics Array) is the older analogue video socket.

The fetch stage

• Once per instruction the processor runs the fetch-execute cycle 取指-执行周期. Fetch comes first, and the exam wants it in register transfer notation 寄存器传送记法.

• A read signal travels on the control bus, the address on the address bus, and the instruction comes back on the data bus. The PC is incremented straight after its address is copied, so that a jump executed later can still overwrite it.

PC → MAR → memory → MDR → CIR, with the PC incremented on the way

Decode, execute, repeat

• Decode: the control unit works out from the instruction in the CIR which operation it is and where its operands are.
• Execute: arithmetic and logic go to the ALU with the result in the ACC; a load or store moves data between memory and a register; a jump writes a new address into the PC.
• Then the cycle repeats from MAR ← [PC]. At the end of every cycle the processor also checks whether an interrupt is waiting.

One LOAD and one ADD walk the registers: fetch, decode, execute, repeat

Worked example: executing two instructions in notation

Interrupts

• An interrupt is a signal that pauses the normal cycle so the processor can deal with an urgent event.
• Causes: a hardware device (a key pressed, a printer buffer empty, a network packet arriving), a software fault (division by zero, an illegal instruction, arithmetic overflow), the operating system's timer marking the end of a time slice, a power-failure warning.
• Interrupts let the system respond promptly without the processor constantly checking devices, and they are how the operating system multitasks.

The cycle, with the interrupt check at the end of each pass

Handling an interrupt

• Finish the current instruction. Save the state: the contents of the PC and the other registers go onto the stack 栈.
• Load the address of the interrupt service routine 中断服务程序 (ISR) into the PC and run it. The ISR handles the event.
• Restore the saved state from the stack and carry on with the interrupted program exactly where it left off.

Save, service, restore, resume

Worked example: the four-mark interrupt answer

Marks that slip away

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