Operating System

• Batch Processing


  Performs many similar tasks for jobs serially without hten eed for user intervention. Each job will only differ in the data it uses.
• Properties of Batch Processing


  - Performs a large number of similar tasks or jobs

  - Can run without user intervention (other than to possibly start it)

  - Can run during out-of-office hours (so that there is less disruption)
• If an error occurs during batch processing, what will occur?


  The error will be reported to a log file, before continuing the batch process.
• Only when will the batch process stop?


  If there is a serious error, where it will require user input.
• In order for a batch process to work, what will it require?


  - A batch or master file
• Batch/Master File


  - Data is gathered over a period and is stored into a batch file for later processing.

  - This file will generally be very large, meaning adding data will become slow.
• Transaction/Temporary File


  Collects data over a shorter period of time, before being merged with the master/batch file at a later date.
• Single-user OS


  One active user at a time (Windows, Playstation OS, IOS)
• How does a single-user OS manage different users?


  Creates home directories for each user, containing users documents, applications settings and browser history.

  These are protected by the OS so that only the user and the administrator can have access to them.
• Multi-user OS


  Allows user to login remotely, giving each user their own display, mouse and keyboard, but is using internal hardware of the server to which their computer is connected.

  Although these devices can use their own resources to run applications, they will connect remotely to the server and run applications there.
• What can terminals take the form of, on a multi-user OS?

  Any networked device.
• Multi-tasking


  The process of swapping applications in and out of the CPU, giving the illusion that they are running in parallel.
• How is multi-tasking implemented?

  As computers are serial machines, meaning they can do one thing at a time, the operating system ensures that each process is swapped in and out so that the user is under the impression that all processes are running at the same time.
• When can the illusion of performing multiple tasks at once break?


  If the process takes a lot of processing time or crashes, as the user may then see the other process is also hanging or becoming unresponsive.
• Multi-programming


  Stores more than one job in the computer's memory at the same time. These jobs appear to be processed by the computers CPU simultaneously.
• How does multi-programming optimise the use of the CPU?

  Prevents it from being idle while waiting for a slower peripheral. The operating system may move jobs in and out of memory and allow each job predetermined time slice to access the CPU.
• What is the method of multi-programming optimising the use of the CPU known as?

  Schedueling
• What is scheduling controlled by?

  The scheduler programme.
• Real-time OS


  Must respond immediately to input provided to the system.
• Why would Windows not be considered a real-time OS?

  Due to the speed of modern OS it is feasible to develop software that can respond to user requests so quickly it seems immediate, however a few seconds have passed.
• Why is a real-time OS used?

  If a decision or action is delayed because of software running slowly, disaster can occur, depending on how this is used.
• How is process scheduling handled?

  By the operating system, deciding when a process will run and for how long.
• What must process scheduling also respond to?

  Input devices and other system wide events.
• What are the signals, making aware of the needed responses, which are sent from the hardware to the CPU, known as?

  Interrupts
• When an interrupt occurs, what will the CPU do on the next instruction cycle?

  Automatically run special OS code that responds to the input.
• Why is it important for the operating system, rather than any other software to handle the event?

  The OS will have the best idea of the overall context of what the user is doing.
• When an interrupt occurs, what does the CPU have a role of?

  Informing the OS rather than dealing with the interrupt itself. The operating system, if it chooses, can ignore interrupts.
• List of common interrupts.

  - Hard drive ready for more data to save, or it has retrieved requested data.

  - Timer interrupt has occurred.

  - Key has been pressed.

  - Peripheral requires more data.

  - Power button has been pressed.

  - Hardware failure or problem encountered.

  - If software needs to sent an urgent message to the OS.
• Buffer


  Small block of memory inside hardware devices and hard drives where work currently being handled by that device is stored.
• If the OS is not ready to deal with an interrupt, what can occur?

  Instead of losing the input, the buffer stores the values until the OS is ready.
• If the buffer overflows, what occurs?

  The motherboard warns the user by soundng a beep.
• What is the 'handing off' of tasks knownas?

  Spooling
• How is spooling beneficial?

  Improves system performance by allowing the CPU to work on other tasks.
• Very specific algorithm used to deal with buffers and interrupts when saving data to a slower device.


  - A chunk of data is sent by the CPU to the device and stored in a buffer.

  - The device will slowly save the data while the CPU is free to do other tasks.

  - When the buffer is empty, an interrupt is sent to the CPU.

  - At the start of the next clock cycle the CPU processes the interrupt by informing the OS.

  - More data is sent to the device and the cycle repeats. This will continue until all data has been saved.
• Double buffering


  More than one buffer is used at once.
• How does double buffering work?

  If one buffer accepts data while the other is used for processing, the OS writes data to the first buffer and the device uses the second for processing. One processed, the OS copies the first buffer into the second and the interrupt is sent.
• Benefit of double buffering

  No delay while an interrupt is being processed, giving the device a much higher active time.
• Ping-pong buffering


  Another version of double buffering where there is no copying between buffers, simply reversing roles after every write.
• What is RAM shared in and managed by?

  Processes, and managed by the operating system.
• What must the OS have to allow management of the system at any given moment?

  Kernel