1.2.1 Systems Software

Operating Systems

A collection of programs that work together to provide an interface between the user and computer

What features do operating systems provide?

Memory management

Resource management

File management

Input/output management

Interrupt management

What is paging?

When memory is split up into equal-sized sections known as pages

What is segmentation?

Splitting up memory into logical sized divisions, known as segments, which vary in size

Virtual memory

A section of the hard drive acting as RAM when space in main memory is insufficient

What is stored in virtual memory?

Sections of programs that aren't currently in use. Moved through paging

Issue with virtual memory

Disk thrashing - computer freezes as a result of pages being swapped too frequently between hard disk and main memory

Interrupts

Signals generated by software or hardware to indicate to the processor that a process needs attention

Interrupt service routine

CPU checks the contents of the interrupt register at the end of each fetch-execute cycle. If there is an interrupt of a higher priority than the current process, the contents of the registers are temporarily copied to a stack and the CPU carries out the interrupt. The interrupt queue is checked for further interrupts before the contents of the stack are popped back into the registers.

Scheduling

Ensures all sections of programs being run receive a fair amount of processing time

Pre-emptive scheduling

Jobs are actively made to start and stop by the operating system

Non pre-emptive scheduling

Once a job is started, it is left alone until it is completed

Round robin

Each job is given a section of processor time (time slice). Once each job in the queue has used its first time slice, the operating system grants each job an equal slice of processor time. This continues until a job has been completed, at which point it is removed from the queue

First come first served

Jobs are processed in chronological order by which they entered the queue

Multilevel feedback queues

Makes use of multiple queues, each of which are ordered based on a different priority

Shortest job first

The queue storing jobs to be processed is ordered according to the time required for completion, with the longest jobs being serviced at the end

Shortest remaining time

The queue storing jobs to be processed is ordered according to the time left for completion, with the jobs with the least time to completion being serviced first

Distributed operating system

OS that runs across multiple devices, allowing the load to be spread across multiple computer processors when a task is run

Embedded operating system

Built to perform a small range of specific tasks, and catered towards a specific device

Multi-tasking operating system

Enable the user to carry out tasks seemingly simultaneously by using time slicing to switch quickly between programs and applications in memory

Multi-user operating system

Multiple users make use of one computer, typically a supercomputer, within a multi-user system

Real time operating system

Designed to perform a task within a guaranteed time frame

BIOS

Basic Input Output System. The first program that runs when a computer system is switched on

BIOS responsibilites

POST (power-on self test) ensures all hardware works correctly

Checking the CPU clock, memory and processor is functional

Testing for external memory devices connected to the computer

Device drivers

Computer programs provided by the operating system and allow it to interact with the hardware

Virtual machines

A theoretical computer in that it is a software implementation of a computer system. Provides an environment with a translator for immediate code to run

Intermediate code

Code that is halfway between machine code and object code. Independent of the processor architecture so can be used across different machines and operating systems

Uses of virtual machines

Testing programs on different operating systems

Protection from malware

Running incompatible software