Cambridge A-Level Computer Science 9618 – 3.1 Computers and Their Components3.1 Computers and Their Components
Learning Objective
Show understanding of the need for input, output, primary memory and secondary (including removable) storage.
Why Computers Need Different Components
- Input devices – allow users and other systems to provide data and instructions to the computer.
- Output devices – present processed information in a form usable by humans or other systems.
- Primary memory (RAM) – stores data and instructions that the CPU needs quickly while a program is running.
- Secondary storage – retains data permanently, even when power is removed; includes both fixed and removable media.
Input
Input devices convert real‑world phenomena into binary data that the computer can process.
- Keyboard – textual data
- Mouse / Touchpad – positional data
- Scanner – optical data → digital image
- Microphone – analog sound → digital audio (via ADC)
- Sensors (temperature, pressure, etc.) – physical measurements → digital values
Suggested diagram: Flow of data from a sensor through an ADC into primary memory.
Output
Output devices transform binary results back into a human‑readable or machine‑usable form.
- Monitor – visual display (pixels)
- Printer – paper output (text, graphics)
- Speaker – audio output (via DAC)
- Actuators – mechanical movement (e.g., robot arm)
Suggested diagram: CPU → RAM → GPU → Monitor.
Primary Memory (RAM)
Primary memory, usually volatile Random‑Access Memory (RAM), provides fast read/write access for the CPU.
| Characteristic | Explanation |
|---|
| Volatility | Data is lost when power is removed. |
| Speed | Typical access time \$< 100\$ ns, much faster than secondary storage. |
| Random access | Any location can be accessed directly, unlike sequential access on tapes. |
| Capacity | Usually measured in gigabytes (GB) for modern PCs. |
Primary memory holds:
- The currently executing program (machine code).
- Variables and data structures used by the program.
- Operating system kernel components that need rapid access.
Secondary Storage
Secondary storage provides non‑volatile, larger‑capacity storage for programs, files, and system data.
| Type | Typical Capacity | Typical Access Time | Removable? |
|---|
| Hard Disk Drive (HDD) | 500 GB – 4 TB | \overline{5}–10 ms | No |
| Solid‑State Drive (SSD) | 256 GB – 2 TB | \overline{0}.1 ms | No |
| Optical Disc (CD/D \cdot D/Blu‑ray) | 700 MB – 100 GB | \overline{0}.2 s | Yes |
| USB Flash Drive | 4 GB – 1 TB | \overline{0}.1 s | Yes |
| Magnetic Tape | Several TB (compressed) | \overline{10} s (sequential) | Yes |
Key reasons for secondary storage:
- Persistence – data remains after power‑off.
- Capacity – far larger than RAM, suitable for archives, media libraries, databases.
- Cost – lower cost per gigabyte compared with RAM.
Relationship Between the Components
The typical data flow in a computer system can be summarised as:
- Input device captures real‑world data.
- Data is transferred via I/O controllers into primary memory.
- The CPU fetches instructions and data from RAM, processes them, and writes results back to RAM.
- When results need to be retained long‑term, they are written to secondary storage.
- For user interaction, the CPU reads results from RAM and sends them to output devices.
Mathematically, the throughput of a system can be expressed as the minimum of the individual component rates:
\$\$
\text{Effective Throughput} = \min\bigl(R{\text{CPU}}, R{\text{RAM}}, R{\text{I/O}}, R{\text{Storage}}\bigr)
\$\$
where \$R\$ denotes the data transfer rate (bits per second) of each component.
Key Points to Remember
- Input and output are the interfaces between the computer and the external world.
- Primary memory provides fast, temporary storage for active data and instructions.
- Secondary storage provides large, permanent storage; some forms are removable for portability.
- System performance depends on the balance between the speeds of these components.