4.20 - Role of an Operating System

What is an operating system?

A program or set of programs that manages the operations of the computer for the user. It acts like a bridge between the user and hardware.

What are the roles of the operating system?

• Memory Management

• Processor Scheduling

• Backing Store Management

• Management of all input and output

• Hides the complexities of managing and communicating with its hardware from the user via an API

What is meant by memory management?

• Allocation - assigning blocks of memory to programs when they are launched

• Deallocation - freeing memory back when a program closes so it can be reused

• Keeping track of which areas of memory are currently in use and which are free

• Preventing conflict - ensuring programs cannot access or overwrite each other's memory space

• Virtual memory - using secondary storage as an extension of RAM when physical memory is full, though this is slower

What does the operating system if RAM becomes full?

• The OS moves data from inactive programs in RAM to an area of secondary storage called virtual memory

• This process is called paging - data is moved in fixed-size chunks called pages

• When the inactive program is needed again, its pages are swapped back into RAM

• Performance consequence - secondary storage is much slower than RAM, so excessive use of virtual memory causes significant slowdown, known as thrashing

What is meant by processor scheduling?

The process of determining the order and duration for which processes are allocated CPU time

What are the goals of processor scheduling?

• Maximise throughput - complete as many processes as possible in a given time

• Fairness - ensure all processes and users receive appropriate CPU time

• Minimise response time - particularly important for interactive systems

• Maximise resource utilisation - keep hardware components as busy as possible

• Prioritisation - allow higher priority processes to access the CPU first

What is meant by Backing Store Management?

The OS process of managing secondary/backing storage (e.g. hard drives, SSDs)

What are the responsibilities of Backing Store Management?

• File management - creating, deleting, reading and writing files

• Space allocation - keeping track of which areas of the disk are in use and which are free

• Directories - organising files into a hierarchical folder structure so they can be located efficiently

• File permissions - controlling which users or programs can read, write or execute files, particularly important in multi-user systems

• Fragmentation management (HDDs Only) - over time files become fragmented across the disk, reducing read/write speed. The OS manages defragmentation to keep files contiguous

What is meant by Peripheral Management?

The OS managing communication between the CPU and peripheral devices.

What are the responsibilities of Peripheral Management?

• Allocation - assigning peripherals to processes without causing conflicts

• Device drivers - using driver software to translate OS commands into device-specific instructions

• Spooling - queuing requests for a peripheral (e.g. printer) when multiple processes need it simultaneously, handling them one at a time

• Error handling - detecting and reporting peripheral errors (e.g. printer out of paper)

What is an interrupt?

A signal sent to the CPU by hardware or software requesting immediate attention.

What happens when the CPU receives an interrupt?

• Finishes its current instruction

• Saves its current state to the stack

• Identifies the interrupt and runs the appropriate Interrupt Service Routine (ISR)

• Restores its previous state from the stack and resumes the original task

What is an Interrupt Service Routine (ISR)?

• A small program run by the OS in response to a specific interrupt

• Each type of interrupt has its own corresponding ISR

• Once the ISR completes, control is returned to the original process

How are interrupts prioritised?

• Every interrupt is assigned a priority level

• Higher priority interrupts can interrupt a currently running ISR

• Lower priority interrupts are queued until the CPU is free

Give examples of hardware and software interrupts:

• Hardware: keyboard input, mouse click, printer error, power failure

• Software: division by zero, program requesting OS services, timer expiry