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Cambridge IGCSE Computer Science · 0478

Topic 4: Software — Part 1

Operating Systems & Interrupts

Classification of software

Software is categorized by whether it serves the computer or the user.

Feature System Software Application Software
Definition Provides the services the computer requires to manage hardware and run apps. Provides the services the user requires to perform specific tasks.
Examples OS, utility programs (antivirus, backup), device drivers, firmware. Word processors, web browsers, games, media players.
Interaction Operates in the background. Interacted with directly by the user.

The hierarchy of running applications

A specific set of layers is required for a computer to execute application software:

  1. Hardware: the physical components (CPU, RAM) that understand only machine code.
  2. Firmware: built-in software in ROM. The bootloader (part of firmware) runs first to check hardware and load the OS into RAM.
  3. Operating System (OS): runs on top of the firmware to provide a platform for apps.
  4. Applications: launched on the OS, which acts as a bridge between the app and the hardware.

Role and functions of the operating system

The OS is essential; a computer is unusable without it. Its nine key functions include:

  • Managing files: creating, naming, and retrieving data; tracking storage locations to prevent overwriting.
  • Handling interrupts: managing signals to ensure the CPU deals with urgent events quickly.
  • Providing an interface: allowing user interaction via GUI (windows, icons) or CLI (text-based commands).
  • Managing peripherals and drivers: using drivers to allow hardware to communicate with the OS regardless of manufacturer.
  • Managing memory: deciding where data is stored in RAM and preventing programs from overwriting each other's space.
  • Managing multitasking: switching CPU attention rapidly between tasks to make them appear to run simultaneously.
  • Providing a platform for applications: allowing developers to write software for the OS instead of specific hardware.
  • Providing system security: controlling user permissions, passwords, firewalls, and encryption.
  • Managing user accounts: creating separate settings and access levels (e.g. Administrator vs Standard) for different users.

Interrupt handling

Definition
A signal sent to the processor indicating an event needs immediate attention.
Generation
  • Hardware: pressing a key, moving a mouse.
  • Software: division by zero errors, or multiple processes trying to access the same memory location.
Interrupt Service Routine (ISR) process
  1. The CPU finishes its current instruction.
  2. The CPU saves the current state (registers and PC) to memory.
  3. The correct ISR is identified and run to handle the event.
  4. The CPU restores the saved state and continues the original task.

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