1.2.1 Systems Software

• Operating System is a program that manages the computer and controls the computer's resources, loaded when the computer boots up
• Operating System Provides

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

  • Paging - memory split up into equal sized pages which can be switched between main memory and hard drive when needed
  • Segmentation is splitting up of memory into logical sized divisions which vary in size
  • Virtual Memory is using secondary storage as if it was RAM. Programs which aren't in use can be moved into secondary storage and moved back once needed in order to free up space in RAM
  • Disk thrashing - an issue with these where the computer freezes as a result of pages being swapped too frequently between the hard disk and memory
• Resource Management - Scheduling

  
  

  Core function of the OS is to ensure all jobs being performed are to receive a fair amount of processing time
• User Interface - The way in which the computer and user interact.
• Interrupt - a signal sent to a CPU to signify another process is in need of processing time
• ISR Interrupt Service Routine

  The program code that is called when an interrupt is triggered.

  Each interrupt has a specific ISR associated with it.

  Relevant ISR is loaded and carries out actions required
• Scheduling - The method an operating system uses to ensure all processes get sufficient processing time.
• Round Robin:

  • each process is given a fixed amount of time. (time slice)
  • If it hasn't finished by the end of that time period, it goes to the back of the queue so the next process in line can have its turn
  • longer jobs will take much longer to complete due to execution being inefficiently split up into multiple cycles
  • doesn't take job priority into account
• First come first serve (FCFS) -

  like queuing in a shop

  • the first process to arrive is dealt with by the CPU until it is finished. Meanwhile, any other processes that come along are queued up waiting for their turn.
  • jobs processed in chronological order
  • straightforward to implement
  • doesn't take job priority into account
• Shortest job first (non-preemptive)

  • picks the job that will take the least amount of time to complete.
  • Normally the algorithm will need to know the time each job will take in advance but this is not always possible
  • suited to batch systems - where shorter jobs are given preference in order to minimise waiting time
  • risk of processor starvation where a particular process does not receive enough processor time in order to be executed and completed
• Shortest remaining time (pre-emptive)

  • scheduler estimates how long each process will take.
  • picks one that takes least amount of time to complete
  • also a risk of processor starvation for longer jobs if short jobs are added to the job queue
• Pre-emptive - The scheduling in which a running process can be interrupted if a high priority process enters the queue and is allocated to the CPU
• Multilevel feedback queues -

  uses a number of queues

  each queue has different priority

  algorithm can move jobs between these queues
• Distributed Operating System

  • run across multiple devices, allowing the load to be spread across multiple computers when a task is run
  • The workload can be shared across a collection of computers using a distributed OS
• Embedded OS

  • Built to perform a small range of specific tasks, catered towards a specific device.
  • An embedded system is a computer that forms part of a device such as a washing machine or vending machine, coffee machine
  • limited in their functionality
  • hard to update although they consume significantly less power than any other OS type
• Multitasking Operating System

  • allows user to carry out tasks simultaneously
  • this is done by time slicing to switch quickly between applications and programs in memory

  e.g Windows, Linux, MacOS

  Modern day operating systems
• Multi-user operating system:

  • Allows multiple users to use a system and its resources simultaneously
  • Used on larger and more powerful mainframe computers able to service large numbers of users at one time (supercomputers)
  • scheduling algorithm must be used to ensure processor time is shared fairly between jobs

  e.g banks and retailers - able to process large number of transactions while being accessed by employees and customers.
• Real-time operating system

  • Designed to carry out actions within a guaranteed time frame even when left running for long periods
  • used in time-critical computer systems, crucial for safety
  • Usually expected response time is within a small fraction of a second

  e.g plane
• Device Driver

  A piece of software that tells the operating system how it can communicate with hardware.

  Used so devices with different models and manufacturers are able to communicate with one another
• Virtual Machine

  • A program that has the same functionality as, and can be used in place of, a physical machine
  • used to create a development environment for programmers so they can test program on different operating systems
  • saves time and money of having to purchase multiple devices solely for testing
  • however running intermediate code in a virtual machine can also be considerably slower compared to the real thing
• Virtual Memory

  • allocated area of secondary storage where pages of inactive jobs are swapped between VM and RAM to free up space in RAM
• Interrupt Management

  • stored in order of priority
  • stored in a priority queue
  • in a special register called the '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 with a higher priority than the process currently being executed, the current contents of the SP registers are temporarily transferred into a stack
  • The processor then loads the appropriate ISR into RAM (every interrupt has a different ISR)
  • A flag is then set to signal the ISR beginning
• Non pre-emptive Scheduling Algorithms:

  1. once a job is started, it is left alone until it is completed
  2. examples: first come first served, shortest job first
• Pre-emptive Scheduling Algorithms:

  1. jobs are actively made to start and stop by the OS
  2. examples: multilevel feedback queues, shortest remaining time, round robin
• Basic Input Output System (BIOS):

  • the first program that runs when a computer system is switched on
  • PC points to the location of the BIOS each start-up
  • Responsible for:
  • POST (Power-on self test): ensures all hardware are correctly connected and functional
  • Checking CPU clock, memory, processor is operational
  • Testing for external memory devices connected to the computer
• Uses of Virtual Machines:

  • Protection from malware:
  • malware will affect the virtual machine rather than the device being used
  • Running incompatible software
  • programs specific to different OS can be run with a VM which saves time and money
  • common example is game consoles being implemented on PCs via a virtual machine