OCR AS Computing - Operating Systems

OCR AS Computing - Operating Systems

1. Functions of Operating Systems

   • A program that manages the operations of a computer for the user
   • Provides a simple interface to remove the complexity of communication to the hardware
   • Memory management
     • An allocation of a specific area of memory whilst the computer is running
     • Store applications in memory simultaneously
     • Paging
       • Memory is divided into fixed size pages
       • Processes may be held in several non-contiguous pages
       • Physical division of memory
       • Stored on disk
     • Segmentation
       • Logical division of address space into varying length segments
       • Stored on disk
     • Virtual Memory
       • The use of secondary storage to make room for the next job which has a share of processor time
       • If there is insufficient RAM to support temporarily inactive jobs
     • If more time is spent swapping pages in and out of RAM instead of executing, disk thrashing occurs and slows down the computer
   • Interrupt service routines
     • Interrupts
       • A signal from a software, hardware or internal clock to the CPU
         • Software interrupts occur when an application terminates or requests certain services
       • Allows multitasking to occur
     • When an interrupt is received, the execution of the running program is suspended and all interrupts of lower priority are disabled
       • Then puts the value of the PC and of each register onto the system stack
         • An interrupt service routine is called
     • Different levels of priority
       • High priority: power-fail interrupt
       • Medium priority: Clock interrupt
       • Low priority: I/O signal
     • Involvement in FDEC
       • A test for an interrupt is carried out at the end of each FDEC cycle
   • Processor Scheduling
     • Scheduler
       • Maximise throughput
       • Be fair to all users
       • Provide acceptable response times
       • Ensure resources are kept as busy as possible
     • Scheduling Methods
       • Round Robin
         • Uses FIFO
         • Each process is allocated a time slice
           • If the process is not executed in this time, the CPU is given to the next process
           • Uses an interval timer to generate interrupts at specific times
         • Guarantees a reasonable response time
       • First come, first served
         • No priority
       • Shortest remaining time
         • The process with the smallest estimated time to completion is run next
         • Reduces the number of waiting jobs and small jobs behind big jobs
         • Requires knowledge of how long a process will take
       • Shortest job first
         • Process with the smallest estimated running time is run next
         • Reduces the number of waiting jobs and small jobs behind big jobs
       • Multi-level feedback queues
         • Gives preference to short jobs and I/O processes
         • Separates processes into categories based upon their processor needs
         • Jobs can move between several queues depending on how much time they are taking up
         • Maximise processor use
   • Backing Store Management
     • When files are loaded, they are transferred from backing storage to memory
       • A directory of where files are stored so that they can be quickly accessed
     • Enables files to be moved, copied, deleted, protected
     • OS needs to know what areas are free to be able to create new files
   • I/O Management
     • Buffer
       • Compensates for the difference in speed between an output device and CPU
     • Communication between the correct input and output devices

2. Types of Operating System

   • Distributed Operating Systems
     • Parallel processing
     • Spreads the workload over multiple computer servers
       • A single job is split into several tasks, each of which is run on separate computers
   • Multi-tasking and Multi-User Systems
     • The ability to run multiple tasks at the same time
     • Multiple users which use multiple tasks, connected to a mainframe CPU and allocated processor time
   • Mobile Operating Systems
     • Multi-tasking computer with its own OS
     • Two operating systems
       • Main system operating the user interface and applications
       • Secondary, low-level real-time OS which operates hardware and radio
   • Embedded and Real Time Operating Systems
     • Minimal user interface (e.g. buttons or a small screen)
     • Accepts input from sensors, and outputs to control devices
     • Safety-critical environments, such as aeroplanes
       • Must respond very quickly to any input
       • Must be able to deal with many inputs simultaneously
       • Fail-safe mechanisms
       • Incorporate redundancy - has backup hardware in the event of hardware failure
   • BIOS
     • Stored in EPROM (Erasable Programmable Read Only Memory)
     • Initialise and test the system hardware and to load the OS into RAM
   • Device Drivers
     • A program that provides a software interface to a particular hardware device
     • Allows OS to access hardware functions without knowledge of the hardware being used
     • Hardware dependent and OS specific
   • Virtual Machines
     • Any instance where software is used to take on the function of a machine including executing intermediate code or running an operating system within another.
     • An emulation of different hardware