ส่วนประกอบคอมพิวเตอร์และระบบฝังตัว

The hundred computers you never see

• A new car carries around a hundred small computers. None has a keyboard or a screen, and most will never receive a software update.
• One runs the engine, one the brakes, one each window. Each does a single job for the life of the car.
• They are embedded systems 嵌入式系统, and there are far more of them in the world than laptops and phones put together.
• Before we get to them, we need the four blocks that every computer, hidden or not, is built from.

The four blocks

• Input devices 输入设备 get data in: keyboard, mouse, microphone, scanner, sensors.
• Output devices 输出设备 give results out: monitor, speakers, printer, actuators.
• Primary memory 主存储器 is the fast memory the processor reaches directly: RAM 随机存取存储器 and ROM 只读存储器. It holds the running program and its data.
• Secondary storage 辅助存储器 is slower but larger, and keeps programs and data when the power is off: hard disk, SSD, optical disc, USB stick.

A monitor: the most familiar output device

Why each block is needed

• Input: the computer can only work on data and instructions that have been entered.
• Output: results have to be presented in a form people, or other machines, can use.
• Primary memory: the processor can only execute instructions and use data held in memory it can address directly, and it must reach them fast.
• Secondary storage: primary memory is small and volatile 易失性, so programs and data must survive between sessions and when the power is off.

Primary against secondary

• Primary memory is measured in gigabytes and answers in nanoseconds; secondary storage is measured in terabytes and answers in microseconds or milliseconds.
• RAM is volatile: its contents vanish without power. Secondary storage is non-volatile.
• The processor cannot run a program straight from a disk. The program is loaded from secondary storage into RAM first, and only then executed.

RAM: fast, volatile primary memory

A hard disk: slow but large, non-volatile secondary storage

Removable storage

• Removable storage 可移动存储 is secondary storage that can be taken out of one computer and put into another: USB flash drives, memory cards, optical discs, external drives.
• It is needed to move data between machines that are not networked, to keep a backup away from the computer, and to distribute software or media.
• Its drawbacks are the reasons for the rest of the syllabus's security topic: it can be lost, stolen, or carry malware.

Optical discs: removable secondary storage read by a laser

Buffers

• A buffer 缓冲 is a temporary area of memory that holds data while it passes between two devices working at different speeds.
• A printer buffer accepts a whole document in a moment, so the processor can carry on with other work while the printer takes minutes to print it.
• A video stream buffers a few seconds ahead, so a brief slowdown in the network does not stop the picture.
• The exam asks why: the two devices work at different rates, and the buffer lets the faster one continue instead of waiting.

Embedded systems

• An embedded system is a computer built into another device to do one fixed job: a washing machine, a microwave, a car's engine controller, a thermostat.
• Its program is stored in ROM as firmware 固件 and is usually never changed.
• Inputs are sensors and buttons; outputs are motors, heaters and small displays. There is no general-purpose operating system and no user installing software.

Worked example: benefits and drawbacks in exam form

Worked example: choosing components for a scenario

Marks that slip away

You've got it