3.1 Computers and their components 计算机及其组成部分

Syllabus learning goals 考纲学习目标

完成本节后，学生需要能够：

• 解释 input、output、primary memory 和 secondary storage 的需要。
• 识别 embedded system 的特征，并说明 benefits 与 drawbacks。
• 描述 syllabus 指定硬件设备的 principal operations。
• 解释 buffer 如何协调不同速度的设备。
• 比较 RAM/ROM、SRAM/DRAM、PROM/EPROM/EEPROM。
• 区分 monitoring 与 control system，解释 sensor、actuator 和 feedback。

Hardware 的 Describe 题通常每个动作对应一分。按信号或数据经过硬件的顺序写独立 bullet points，不要只列零件名称。

1 Input, output, primary memory and secondary storage

Input and output

• Input device 把外部 data 或 instructions 送入系统。Sensor 把 temperature、pressure、sound 等物理量转换为系统可处理的数据。
• Output device 把处理结果传给用户或环境。Screen、speaker、printer 呈现信息；actuator 产生物理动作。

Primary memory

Primary memory 由 CPU 直接访问，保存当前使用的 instructions 和 data，例如：

• Program instructions currently being executed。
• Current sensor readings。
• Current/recent video or audio data。
• Intermediate results。
• Start-up instructions stored in ROM。

Secondary storage

Secondary storage 长期保存 programs and data，通常是 non-volatile。

场景判断：current journey mileage 可在 primary memory 临时保存；total mileage since the car was built 必须断电后保留，应放在 secondary storage。当前处理的视频帧可在 primary memory，完成后的视频文件放在 secondary storage。

2 Embedded systems 嵌入式系统

Embedded system 是构成较大设备一部分、执行单一任务或有限功能的计算机系统。

Mark-secure characteristics

• Dedicated to a single task or a limited number of functions.
• Built into / integrated into a larger system.
• Contains a processor, memory and input/output capability。
• Often uses dedicated hardware and firmware。

Applied answer

A washing-machine controller is an embedded system because it is built into the washing machine and is dedicated to controlling wash cycles. It receives input from buttons and sensors and controls the motor and valves.

Benefits

• Hardware/software can be optimised for one task。
• May need less processing power, memory and storage。
• Can be small, low-cost and energy-efficient。
• Performs the task automatically and consistently。

Drawbacks

• Difficult for the user to change or update the firmware.
• Functionality cannot easily be extended or adapted for another task。
• Troubleshooting, repair and upgrading may require a specialist。
• Faulty/outdated devices may be discarded, leading to e-waste。

3 Principal operations of hardware devices

Laser printer

• Print instructions/data enter a buffer。
• A photosensitive drum receives an electrical charge。
• A laser changes the charge at selected positions, forming an electrostatic image。
• Charged toner is attracted to the required drum areas。
• Paper attracts the toner from the drum。
• Heated fuser rollers use heat and pressure to fix toner to paper。

3D printer

• A digital model is created with CAD/3D-modelling software and divided into layers。
• The printer uses additive manufacturing and builds one layer at a time。
• The process repeats until all layers form the object。
• Material cools/sets or resin is cured with UV light。

Microphone and speakers

Microphone：

• Sound waves vibrate a diaphragm。
• The vibration becomes a varying analogue electrical signal。
• An ADC samples it and converts it to digital values。

Speakers：

• A DAC converts digital samples into an analogue signal, which may be amplified。
• Current changes the magnetic field of a coil。
• The coil moves a cone/diaphragm, producing sound waves。

Magnetic hard disk

• Magnetic platters rotate at high speed on a spindle。
• Surfaces are divided into concentric tracks and sectors。
• An actuator positions a read/write head above the required track。
• Writing current creates different magnetic patterns to represent binary data。
• When reading, magnetic variations induce electrical variations in the head and are decoded as data。

Solid-state (flash) memory

• Data is stored as electrical charge in floating-gate/transistor cells。
• Charge states represent binary data。
• A controller locates and reads/writes cells electronically。
• There are no moving parts, giving fast access, low power use and shock resistance。

Magnetic storage often has a lower cost per unit of storage，适合大量视频等高容量场景。Solid-state storage 通常更快、更省电且抗震。Flash cells 会受到 write/erase cycles 限制；选择必须结合 capacity、cost、speed、durability 和题目场景。

Optical disc reader/writer

• The disc spins at high speed and the optical head follows a spiral track from the centre outwards。
• A laser is directed at the rotating disc。
• Reading：pits and lands reflect different amounts of light；a sensor interprets the differences as 0 and 1。
• Writing：a higher-power laser changes/burns selected positions to represent data。

Touchscreen

系统确定 x and y coordinates，再把 coordinate position 映射到对应 menu item/icon。

Virtual reality headset

• One or two screens/lenses provide separate images to the eyes, creating a stereoscopic view。
• A gyroscope/accelerometer detects direction and/or speed of head movement。
• A processor analyses the sensor data and updates the displayed view。
• Digital cameras may record eye movement；speakers provide corresponding sound。

高分答案应形成 movement detected → data processed → image updated 的动作链。

4 Buffers 缓冲区

Buffer 是临时保存数据的 memory area，用于协调速度不同的 devices/processes。

Four-mark answer pattern

The computer and hard disk operate at different speeds. The buffer provides temporary storage. The computer transfers data to the buffer at its higher rate, so it does not have to wait. Data is transferred from the buffer to the hard disk at the disk's slower rate.

应用包括 printer、audio/video streaming、sensor readings 和 camera frames。Buffer 不会让慢设备本身变快；它吸收短期速度差。若输入长期快于输出，buffer 仍会填满。

5 RAM and ROM

ROM 用于 embedded system，因为它保存不常改变的 firmware/start-up instructions，而且 data must be retained without power。RAM 用于可快速改变的 current readings、settings 和 results。

6 SRAM and DRAM

Three-difference answer pattern

• DRAM requires refreshing, whereas SRAM does not.
• DRAM stores a bit as charge in a capacitor, whereas SRAM uses a flip-flop/latch.
• DRAM is slower and cheaper per bit, whereas SRAM is faster and more expensive.

每一分写成 paired comparison；不要只描述一边。

7 PROM, EPROM and EEPROM

Comparison pattern

• EPROM is erased using ultraviolet light, whereas EEPROM is erased electrically.
• EPROM normally has to be removed and erased as a whole, whereas EEPROM can be reprogrammed in circuit and selected data changed.

EEPROM 适合保存可更新 firmware，因为不必拆下芯片即可更新。

8 Monitoring and control systems

Sensor 测量环境并产生 input data，例如 temperature、pressure、infra-red 和 sound sensor。Actuator 根据 system output 产生动作，例如 motor、heater、valve、pump、door mechanism 或 alarm。

Feedback loop

sensor reading -> processor compares -> actuator changes environment
       ^                                      |
       |------------- new reading ------------|

Control-system justification

It is a control system because it acts autonomously on sensor data. It sends output to an actuator to change the environment. This changes the next sensor reading, which is returned as feedback and used to decide the next action.

9 Exam-answer checklist 真题检查表

• Embedded system 是否写了 dedicated function 和 part of a larger system？
• 设备原理是否按动作先后写，而非只列 components？
• Buffer 是否写了 temporary storage、different speeds 和 data flow？
• SRAM/DRAM 是否每一点都是 paired comparison？
• EPROM/EEPROM 是否区分 UV、electrical signal 和是否移出电路？
• Sensor 是否写 measured quantity，actuator 是否写 physical action？
• Control system 是否出现 autonomous action、actuator 和 feedback？
• Explain 题是否形成 cause-and-effect chain？

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本材料依据 9618 syllabus 3.1 的八项要求，并综合本页所列 2021-2025 past-paper questions 及 mark schemes。加粗英文短语优先采用评分方案中反复出现的可得分表述。