O-Level Computing (7155) Computer Systems and Architecture: hardware and software, peripherals, memory and storage, and the CPU and fetch-execute cycle

Why this module matters

Computer Systems and Architecture explains what a computer is made of and how it actually runs the programs you write elsewhere in the syllabus. Knowing how the CPU, memory and storage cooperate helps you reason about why a program runs, why data is lost when the power goes off, and why some machines are faster than others. This is a theory module, examined in the written Paper 1, and it gives you the vocabulary to describe a computer system precisely.

This guide ties together the matching dot-point pages, each with its own worked detail and practice. The strands below build from the broad system view down to the processor.

Hardware and software

The first distinction is between hardware (the physical parts you can touch, such as the CPU, memory, keyboard and monitor) and software (the programs that tell the hardware what to do). See hardware and software for the full distinction and examples.

Software then splits in two. System software manages the computer itself; the central example is the operating system, which manages memory, files, hardware and running programs. Application software is the programs people use to complete a task, such as a word processor or a spreadsheet. A useful summary: system software keeps the machine working, while application software lets the user get a job done.

Peripherals and the input-process-output model

A computer system follows the input-process-output model: it takes in data (input), processes it, and produces a result (output), often with storage holding data for later. See input, output and peripherals for how to classify peripherals.

Peripherals are devices connected to the computer, and they fall into three groups: input devices (such as a keyboard, mouse or scanner) that send data in; output devices (such as a monitor, printer or speaker) that present results; and storage devices (such as a hard disk or USB drive) that hold data. Some devices, such as a touchscreen, can be both input and output.

Memory and storage

The computer holds data in several places that differ in speed, size, volatility and purpose. See memory and storage for the full comparison.

• RAM (random access memory): volatile working memory holding the programs and data currently in use; cleared when power is removed.
• ROM (read-only memory): non-volatile memory holding fixed start-up instructions (such as the bootstrap); not normally changed.
• Secondary storage: non-volatile, large and permanent storage (hard disk drive, solid-state drive, USB drive) that keeps files when the computer is off, but is slower than RAM.

Volatile means the contents are lost without power; non-volatile means they are kept. RAM is fast and temporary, ROM is fixed and small, and secondary storage is large and permanent.

The CPU and the fetch-execute cycle

The central processing unit (CPU) is the part that runs instructions. See the CPU and fetch-execute cycle for its parts and how it works. Its main parts are the control unit (which directs operations and decodes instructions) and the arithmetic logic unit (ALU) (which performs calculations and logical comparisons), supported by small fast stores called registers.

The CPU repeats this cycle billions of times a second. Clock speed (in gigahertz) is how many cycles run per second, so a higher clock speed generally means faster performance. The number of cores is how many independent processing units the CPU has; more cores allow several tasks (or parts of one task) to run at once, which helps multitasking and software written to use them.

How this module is examined

• Paper 1 (written, 60%). Classify hardware and software, define system versus application software, classify peripherals, compare RAM, ROM and secondary storage on volatility and purpose, and describe the parts of the CPU and the stages of the fetch-execute cycle.
• Use precise terms. Marks come from accurate vocabulary: volatile and non-volatile, control unit and ALU, fetch, decode and execute, clock speed and cores.

Check your knowledge

Try these, then take the matching quiz for this module.

1. State the difference between system software and application software, with one example of each. (3 marks)
2. Define volatile, and state whether RAM is volatile or non-volatile. (2 marks)
3. Name the two main parts of the CPU and what each does. (2 marks)
4. List the three stages of the fetch-execute cycle in order. (3 marks)