A Level Computer Science: Operating Systems

• Computer Science A Level: 1.2.1 Systems Software

Operating Systems

• It is a collection of programs providing an interface between the user and the computer
• Enables users to communicate and perform low-level tasks, managing computer memory and resources
• Commonly found in laptops, mobile phones, and game consoles.
• Examples are Windows, macOS, iOS, and Android

Essential Operating System Features

• Memory management (paging, segmentation, virtual memory).
• Resource management (scheduling).
• File management (moving, editing, deleting files and folders).
• Input/Output management (device drivers).
• Interrupt management.
• Utility software (disk defragmenter, backup, formatting etc.).
• Security (firewall).
• User interface.

Memory Management

• Shares memory fairly between programs because main memory is not big enough to store all of them
• Enables efficient sharing through paging, segmentation and virtual memory

Paging

• Memory splits into equal-sized sections called pages
• Programs consist of equally-sized pages, swapped between main memory and the hard disk as required

Segmentation

• Memory splits into logical divisions (segments) of varied sizes
• Segments reflect program structure and flow, allocating code blocks like conditionals or loops

Virtual Memory

• Uses hard drive section as RAM when main memory is full
• Temporarily moves unused program sections to virtual memory via paging, freeing RAM for active programs
• Key issue is disk thrashing

Disk Thrashing

• Computer 'freezes' due to frequent page swapping between hard disk and main memory
• More time is taken to transfer pages than to run the program, worsening as virtual memory fills

Interrupts

• Signals from software or hardware alerting the processor to a process needing attention
• Different interrupts have different priorities considered by the OS when allocating processor time
• Stored by priority in a priority queue within the interrupt register
• Operating system utilises device drivers to ensure interrupts are serviced fairly
• Examples include printer signalling job completion or peripheral power failure

Interrupt Service Routine

• Processor checks the interrupt register's content at the end of each Fetch-Decode-Execute cycle
• For higher priority interrupts, current contents of CPU registers are moved to a stack
• Processor loads the matching ISR into RAM and the flag is set to show the ISR has begun

Interrupt Handling Completion

• After servicing, the flag is reset
• Interrupt queue checks for further higher-priority interrupts
• Processes are repeated until all priority interrupts are serviced or if no higher priority interrupts remain.
• Contents are transferred back into the registers in memory, and the Fetch-Decode-Execute cycle continues

Scheduling

• Guarantees all jobs receive fair processing time

Pre-emptive Scheduling

• Jobs actively start and stop by the operating system like multi-level feedback queues, shortest remaining time, round robin

Non Pre-emptive Scheduling

• Once started, jobs complete without being stopped like first come first served, shortest job first

Scheduling Algorithms

• The scheduling depends on the task.

Round Robin

• Each job receives a time slice to execute
• Once each job uses the first slice, the system grants an equal slice
• It continues until the job completes and exits the queue
• Longer jobs take longer to complete, even though the job is seen to
• The algorithm doesn't consider job priority

First Come First Served

• Jobs process in chronological order
• Although it is implemented straightforward, FCFS doesn't allcoate processor time based on priority

Multi-level Feedback Queues

• Uses multiple queues which orders based different priorities
• It is difficult to implement due to deciding which job to prioritse based on prioritie

Shortest Job First

• The queue stores job to be processed according to the time
• Most suited to batch systems, where shorter jobs prefer to minimise time
• The process has to calculate how long each job takes
• It risks processeor starvation if short jobs go to the queue

Shortest Remaining Time

• The queue stores the time is ordered to the time left for completion
• It also ricks process starvation if short job is added to the queue

Distributed Operating System

• Runs across multiple devices
• It spreads the load across multiple computer process when runninng the operating systems

Embedded Operating System

• Catered towards s specific device to the operating system
• Low funcionality and hard to update
• Limited functionality

Multi-tasking Operating System

• Uses time slicing to switch between in programs and applications in memory

Multi-user Operating System

• Uses algorithm to allocate process

Real Time Operating System

• Used critical time
• Designed to perfrom a task within a time frame

BIOS

• First program that runs when the switch is turned on
• Register points to start-up of the computer
• Checks CPU clock, memory, hardware

Bootstrap

• Loads the operating system from the hard disk into main memory

Device Drivers

• Allows the operating system to interact with the hardware
• Communicates these requests to the operating system and the request is displayed to the screen

Device Drivers (Architecture)

• Because of the archicture, drivers must be used to different types such as game consoles

VM

• It provides an environment with an translator for code

Intemediate Code

• The code is halfway betweem maching ecode
• Is independent, can be used accross operating system

Virtual Machines

• Tests program on operating system
• It saves time, money
• Runs slower compared to running low-level code

Other uses of VM

• Protection from malware: malware run in the VM
• Running incompatible software: run in VM