Software

Cambridge IGCSE Computer Science 0478 is not divided into tiers. Every candidate takes the same two components, both papers assess the full A*–G grade range, and all ten subject-content topics are required. So every one of the 19 requirements mapped in this chapter is examinable for you, whichever grade you are working towards. Nothing here is optional, and nothing here is reserved for a different tier — there is no other tier.

A computer is built in layers, and each layer only works because the layer beneath it is already running. Hardware executes machine instructions and nothing else. Firmware, held in non-volatile memory, starts the hardware and its bootloader loads the operating system. The operating system then manages files, memory, peripherals, users and security, and provides the platform on which application software runs. While all of that is happening, interrupts let an urgent event — a key press, a division by zero — reach the CPU without it having to keep asking. And because a CPU can only execute machine code, every program a person writes must first be put through a translator: an assembler, a compiler or an interpreter.

Topic 4 is assessed in Paper 1: Computer Systems. Every candidate takes Paper 1 and Paper 2 — there is no choice of components and no tier — and both papers assess the full A*–G grade range and all three assessment objectives. Paper 1 is a written paper of 1 hour 45 minutes carrying 75 marks, made up of short-answer and structured questions set on Topics 1 to 6. All questions are compulsory, candidates answer on the question paper, the paper is externally assessed, and calculators are not permitted. Topic 4 contains no calculations at all, so what the paper is really testing here is different: your ability to describe a process in the right order and to compare two things against a stated context.

System software provides the services that the computer itself requires. It manages the hardware, keeps the machine usable, and provides the platform that other software runs on. The two categories the syllabus names are the operating system and utility software. System software is not written to do a job for the user; it is written to keep the computer in a state where a job can be done at all.

Application software provides the services that the user requires. It performs the tasks a person actually wants done — writing a letter, editing a photograph, keeping accounts, browsing the web, designing a part, playing a game. Application software cannot reach the hardware on its own: it depends on the system software beneath it, and requests every file, every allocation of memory and every use of a device through the operating system.

An operating system is system software that manages the computer's hardware and resources and provides a platform on which application software can run. It is not one job but nine, carried out at the same time and constantly interacting: managing files, handling interrupts, providing an interface, managing peripherals and drivers, managing memory, managing multitasking, providing a platform for running applications, providing system security, and managing user accounts. Every one of those functions exists because a shared machine with limited resources needs someone to be in charge of them.