1.2.1 - Systems Software

• Operating Systems (OS)

  • a collection of programs that work together to provide an interface between the user and the computer
  • enable user to communicate with computer and perform certain low-level tasks
  • essential in devices like laptops, phones and consoles
• Operating Systems - Desktop Examples

  • windows
  • macOS
• Operating Systems - phone examples

  • iOS
  • android
• Operating Systems - Features

  • memory management
  • resource management
  • file management
  • input/output management
  • interrupt management
  • utility software
  • security
  • user interface
• Memory Management - Examples

  • paging
  • segmentation
  • virtual memory
• Resource Management - Examples

  • scheduling
• File Management - Examples

  • moving
  • editing
  • deleting files
  • folders
• Input/Output Management - Examples

  • device drivers
• Utility Software - Examples

  • disk defragmenter
  • backup
  • formatting
• Security - Examples

  • firewall
• Memory Management

  • computer memory needs to be shared fairly between programs and applications
  • main memory is often not large enough to store all programs being used
  • OS must determine and allocate length of processor time each task receives via paging, segmentation and virtual memory
• Paging

  • when memory is split into equal sized sections (pages) with programs made up of a number of equally sized pages
  • these pages can be swapped between main memory and hard disk as needed
• Segmentation

  • splitting up memory into logically sized divisions (segments) which vary in size
  • these represent the structure and logical flow of the program
  • segments are allocated to blocks of code like conditional statements or loops
• Virtual Memory

  • uses section of the hard drive to act as a RAM when there isn't enough space in main memory to store programs being used
  • sections of program not in use are temporarily moved to virtual memory through paging, freeing up memory for other programs in RAM
• Disk Thrashing

  • when computer 'freezes' as a result of pages being swapped too much between hard disk and main memory
  • due to this, more time is spent transferring pages between main memory and hard disk than spent running program
  • issue gets worse as virtual memory fills up
• Interrupts

  • signals generated by software/hardware to indicate to processor that a process needs attention
  • different types have different priorities and how urgent they are needs to be taken into account by OS when allocating processor time
  • interrupts are stored in order of priority in abstract data structure - priority queue in interrupt register
• Interrupts - Examples

  • printer signaling a completed print
  • peripheral signaling power failure
• Interrupt Service Routine (ISR)

  • processor checks contents of interrupt register at end of each FDE cycle
  • if interrupt exists that's higher priority than process being executed, current contents of special purpose registers in CPU are temporarily transferred to stack
  • processor responds to interrupt by loading appropriate ISR into RAM
  • a flag is set to signal the ISR has begun, once serviced, the flag resets
• Scheduling - Algorithm Types

  • pre-emptive
  • non pre-emptive
• Pre-Emptive

  • jobs are actively made to start and stop by OS
• Pre-Emptive - Examples

  • Multilevel Feedback Queues
  • Shortest Remaining Time
  • Round Robin
• Non Pre-Emptive

  • once job is started, it's left alone until it is completed
• Non Pre-Emptive - Examples

  • First Come First Served (FCFS)
  • Shortest Job First
• Round Robin

  • each job is given a section of processor time (time slice) that it can execute in
  • when each job in queue has used its first time slice, OS repeats process until a job has been completed, at which point it's removed from queue
• Round Robin - Disadvantages

  • while it ensures each job is seen to, longer jobs take much longer for completion as execution is inefficiently split into multiple cycles
  • algorithm doesn't account for job priority
• First Come First Served (FCFS)

  • jobs are processed in chronological order by when they entered the queue
  • straightforward to implement
  • does not account for job priority
• Multilevel Feedback Queues

  • uses multiple queues, each ordered based on a different priority
  • this can be difficult to implement due to deciding which job to prioritise based on a combination of priorities
• Shortest Job First

  • queue storing jobs is ordered according to time required for completion, with longest jobs serviced at the end
  • most suited to batch systems
  • requires processor to know/calculate how long each job will take which isn't always possible
  • there's also a risk of processor starvation if short jobs continue being added to job queue
• Shortest Remaining Time

  • queue storing jobs is ordered according to time left for completion
  • jobs with least time to completion are serviced first
  • risk of processor starvation for longer jobs if short jobs are added to job queue
• OS - Types

  • distributed
  • embedded
  • multi-tasking
  • real time
• Distributed OS

  • runs across multiple devices
  • allows load to be spread across multiple computer processors when task is run
• Embedded OS

  • built to perform small range of specific tasks
  • catered to a specific device
  • limited functionality
  • hard to update
  • consume significantly less power than other OS
• Multi-Tasking OS

  • let user carry out tasks almost simultaneously
  • uses time slicing to switch quickly between programs and applications in memory
• Multi-User

  • multiple users make use of one computer (usually a supercomputer) in a multi-user system
  • scheduling algorithm must be used to ensure processor time is split fairly between jobs
  • without suitable scheduling algorithm, it risks processor starvation
• Real Time

  • often used in time-critical computer systems
  • designed to perform task in guaranteed time frame
  • used in any situation where a response in a certain time period is crucial to safety
• Processor Starvation

  • when a process is not given enough processor time to execute and complete
• Basic Input Output System (BIOS)

  • first program to run when computer is turned on
  • program counter (PC) register points to location of BIOS at each start up
  • BIOS is responsible for running various key tests before OS is loaded to memory
  • critical to system as only after checks are completed can OS be loaded to RAM from hard disk
• BIOS - Checks

  • Power-on Self Test (POST) - ensures all hardware is connected and functional
  • checking CPU clock, memory and processor is operational
  • test for external memory devices connected to computer
• Device Drivers

  • programs provided by OS and let OS interact with hardware
  • when hardware is used, it's the device driver that communicates the request to OS which produces relevant output
  • device drivers are specific to computer's architecture, so different drivers need to be used for different device types
  • as they interact with the OS, the drivers are also specific to the OS on the device
• Virtual Machine

  • theoretical computer
  • a software implementation of a computer system
  • provides an environment with a translator for intermediate code to run