Operating Systems

Operating systems are a collection of programs that (in brief)

Have Special Permissions: Are the first program to run on a computer, loaded by the ROM BIOS (Basic Input/Output System)
Provide Hardware Abstraction: Provide a interface between most software and hardware
Provide Program Management & Multitasking: Control the execution of other programs
Provide User Management & Multiusers: Provide an interface between the users and the computer / applications
Manage Security: Related to multitasking, multiusers & network security
Provide a User Interface: Either command line or Graphical (GUI)

Modern Operating Systems are usually split into two parts:

The kernel
- the heart of the operating system. It is the first program loaded when the computer starts up, it manages computer resources, and it handles requests from system programs and applications.
System programs
- run on top of the kernel. Used to connect the kernel to user applications and peripheral devices. E.g., Device drivers, file systems, user management programs, networking programs, and system utilities like disk defragmenters.

User applications then run on top of this edifice.

Not all computer systems need operating systems! For example, simple embedded systems and interfaces such as microwave cooker controls, AC remote control etc. These are usually single purpose devices.

Different Operating Systems

There are a large range of Operating Systems Historically - they developed to reflect technology changes and user requirements.

Ones you should know about are: Windows (10), CP/M, DOS, OS2, MacOS (OSX ~ BSD), iOS, Multics, Unix, GNU/Linux, Android

See: Crash Course CS and Wikipedia: Timeline of operating systems for lots more!

Timeline of Consumer facing Operating Systems

The two current major families of Operating Systems are Windows and Unix (from which Linux and MacOS/iOS derive)

Windows Family Tree

Unix, Linux, BSD, Darwin (iOS, OSX), Solaris Tree

Functions of an Operating System

Boot up

BIOS loads from ROM on Motherboard
Operating system stored on secondary storage (HDD/SDD/etc) then loads all other functionality

Managing Programs & Memory & Multitasking

Memory Virtualization
Dynamic Memory Allocation
File Systems & Disk management
Permissions - access memory / files
Interrupt Handling
Network interfaces
CPU management

Managing Users

User Accounts & Security / Permissions
Multiuser Management
User Interface (HCI = Human Computer Interaction)
- Command Line vs Graphical UI (CLI vs GUI)
Interrupt Handling

Security

Access permissions - some programs and files accessible by different users, or only administrators (super users - e.g., sudo)
Memory - control which parts are executable to help prevent malicious software
Network security (see topic 6)

Interrupts

For computers to interact with external devices and to multitask they need to have a way of detecting and responding to events. The events can be triggered by users, hardware, software.

Hardware
- Printer / Disk drive / other interface ready to receive more data (because of internal buffers)
- Error in hardware, e.g., paper jam, out of storage, disconnected peripheral, etc...
- Keyboard / Mouse event - user interaction
- Network interrupt
Software
- Switching between processes in multitasking
- Task finished
- Error / program crashed

Exercises & Exam Questions

Create a mind map of the features of an operating system
Name 4 common operating systems and describe when/where they are used
What features of an operating system are needed to enable multitasking
What features of an operating system are needed to enable multiple users

0478 s18 qp13

0478 m17 qp12

0420 s09 qp1 (but a really common exam question)

0420 s11 qp12

Semi-Official list of OS features (in every textbook / IGCSE CS website)

Provides an interface for computer interaction (CLI / GUI)
Control of inputs and outputs
Interrupt handling
Management of hardware and peripherals
Error handling
Management and loading of software
Management of user accounts
File utilities (copy, save, sort, delete)
Processor management for multitasking
Memory management
Security (access rights, firewalls etc)

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