1.2.1 Systems Software

Operating system

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

Operating system features

Memory management
Resource management
File management
Input/Output management
Interrupt management
Utility software
Security
User interface

Paging

Memory is split up into equal sized sections (pages) that can then be swapped between main memory and the hard disk

Segmentation

Splitting up memory into logical sized divisions (segments

)Virtual memory

Uses a section of the hard drive to act as RAM when the space in main memory is insufficient. Programs that are not currently in use are temporarily moved into virtual memory

What is the issue with paging, segmentation and virtual memory?

They cause disk thrashing. This is when the computer 'freezes' and occurs as a result of pages being swapped too frequently between the hard disk and main memory

Interrupts

Signals generated by software or hardware to indicate to the processor that a process needs attention.
Different types of interrupts have different priorities and they are stored in order of their priority in an interrupt register

Interrupt Service Routine

The processor checks the contents of the interrupt register at the end of each FDE cycle
If an interrupt exists that is of higher priority to the process being executed, the current contents of the special purpose registers in the CPU are temporarily transferred into a stack
The processor responds to the interrupt by loading the appropriate interrupt service routine into RAM

Scheduling

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

Pre-emptive

Jobs are actively made to start and stop by the operating system
e.g. MFQ, SRT, RR

Non pre-emptive

Once a job is started, it is left alone until it is completed
e.g. FCFS, SJF

Round Robin

Each job is given a time slice of processor time within which it is allowed to execute
Once each job in the queue has used its first time slice, the OS again grants each job an equal slice of processor time
This continues until a job has been completed

Advantages of round robin

Ensures each job is seen to

Disadvantages of round robin

Longer jobs will take much longer for completion due to being inefficiently split up into multiple cycles
Does not take into account job priority

First come first served

Jobs are processed in chronological order

Advantages of first come first served

Straightforward to implement

Disadvantages of first come first served

Does not allocate processor time based on priority

Multilevel feedback queues

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

Advantages of multilevel feedback queues

Allocates processor time based on priority

Disadvantages of multilevel feedback queues

Difficult to implement

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

Advantages of shortest job first

Suited to batch systems

Disadvantages of shortest job first

Requires the processor to calculate how long each job will take which is not always possible
Risk of processor starvation

Shortest remaining time

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

Advantages of shortest remaining time

Reduces waiting time

Disadvantages of shortest remaining time

Risk of processor starvation

Processor starvation

When a particular process does not receive enough processor time in order to execute and be completed

Distributed Operating System

Runs across multiple devices, allowing the load to spread across multiple computer processors

Embedded Operating System

Performs a small range of specific tasks

Multi-tasking Operating System

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

Multi-user Operating System

Makes use of one computer within a multi-user system. Therefore a scheduling algorithm must be used to determine ensure processor time is shared fairly between jobs

Real Time Operating System

Used in time-critical computer systems and designed to perform a task within a guaranteed time frame

BIOS

The first program that runs when a computer is switched on

Key tests BIOS runs before the operating system is loaded into memory

• POST (Power-on self test) which ensures that all hardware are correctly connected and functional
• Checking the CPU clock, memory and processor is operational
• Testing for external memory devices connected to the computer

Bootstrap

The program that loads the operating system from the hard disk into main memory

Device drivers

Allow the operating system to interact with hardware

When a piece of hardware is used it is the device driver that communicates this request to the OS And high can then produce the relevant output

Virtual machine

A theoretical computer that is a software implementation of a computer system. It provides an environment with a translator for intermediate code to run

Intermediate code

Code that is halfway between machine code and object code