Operating Systems

Operating Systems

1. Why have one?

   • It manages the computer and all its hardware
     • CPU
     • Multitasking
     • Interrupts
     • Memory
     • Backing Store
     • Peripherals
     • Security
     • Interface

2. CPU

   • Modern processors can execute more instructions than the memory can keep up with
     • While the processor waits for other components to catch up it is not doing anything
       • An OS will make use of this time by allocating CPU time to different processes (running applications)
         • This allows multitasking

3. Multi-tasking

   • Where a a processor appears to be doing two tasks simultaneously
     • In reality the processor runs commands from each task separately, filling in the time when it is waiting for other components
     • The OS has to keep the tasks separate and manage use of the accumulator and memory locations

4. Interrupts

   • A method to immediately pause execution of all processes and execute a new one
     • The OS will manage interrupt requests, backing up existing processes so they can be resumed when the interrupt is finished

5. RAM

   • RAM is split into "pages"
     • Each page is presented to its program as if it is the only memory the system has
       • This prevents programs from snooping on data from other programs

6. Backing Stores

   • Storage devices used to keep backups of important files
     • OS can often manage this for the user
       • It will male regular backups in the right format in an appropriate device

7. Drivers

   • OS use device drivers to communicate to different peripherals
     • A device driver maps the OS's commands to the device's commands

8. Spooling

   • If my program needs to print a document, it tells the OS to print it
     • The OS will spool the print job (keep it in queue until the printer is ready
       • This means my program doesn’t need to worry if the printer is busy or unplugged or out of ink

9. Security

   • OS have a range security features

10. IInterface

    • The mechanism by which the user controls the functions of the computer
      • Most operating systems support two types of interface
        • A GUI (Graphical User Interface)
        • A CLI (Command Line Interface)

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