3.3.1 Operating systems A2 computing OCR

3.3.1 Operating systems A2 computing OCR

1. Purposes of OS's

   • Provides...
     • Interface between apps software and hardware
     • Utility software
     • User interface
     • Scheduling
   • Manages input and output
   • Does file management
   • Manages memory

2. Booting a PC

   • 1. Bios opens
     • Communicates with hardware devices like keyboard
       • Most stored in ROM
         • Some stored in CMOS
           • SO user can change settings (e.g. where to boot from)
     • 2. POST opens (Power on self test)
       • Checks if memory ok
         • If clock ok
           • If timer ok
             • If keyboard present
               • If screen display memory ok
                 • If bios not corrupted
       • 3. Bios initialises video card

3. FAT (File allocation table)

   • Organises system files and clusters
     • Clusters scattered through hard disks
   • Holds...
     • Cluster no.
       • Next cluster in chain
     • Flag to indicate last cluster in chain
     • Flag to indicate bad/unusable clusters
     • Flag to indicate if cluster is free

4. Directory table

   • Holds locations of first cluster of file and...
     • File name
       • File extension
     • File attribute
     • Date of creation
     • File size

5. Interrupts

   • A signal sent from a hardware device or program to signal cpu for time
   • Several types
     • Numbered by priority...
       • 1. PSU interrupts
         • 2. Other hardware interrupts
           • 3. Program interrupt
             • 4. Timer interrupt
               • 5. I/O interupts
                 • Signal the CPU to act on key press or more paper for printer and etc..
               • Forces processor to move to another task
             • Stops processors from carrying out an illegal operation (div by zero)
           • Other hardware failures can also cause data loss
         • Power failures are critical and can cause data loss
   • Stored in a queue
   • Avoid delays
     • Avoids loss of fata
       • Used to obtain CPU time for higher priority task
   • An indicator that a device needs processing

6. Scheduling

   • HLS (High level scheduler)
     • Paces jobs in ready queue that are coming in the system
   • MLS
     • Moves jobs between primary and secondary memory
   • LLS
     • Loads next job into processor once current job is finished
   • Policies
     • RR
       • Time slice allocated to each job
         • Once time slice complete sent back to queue
           • Priority not taken account of
             • Simple though
     • FCFS
       • FIFO structure
       • First job in is the first executed
       • No priority accounted
     • Multi level scheduling queue
     • Shortest job first
       • The job with the least time done first
         • No priority taken account of
   • goals
     • Make efficient use of processor time
     • Make efficient use of resources (memory etc)
     • Maximise amount of users without apparent delay
     • Maximize throughput of CPU
     • To get through as many jobs as possible

7. Memory management

   • Goals
     • allocate memory to allow different processes to run at same time
     • Prevent programs from interfering with each other
     • Reallocate memory when necessary
     • Enable memory to be shared
     • To deal with paging and segmentation
   • Paging
     • Allows porgrams to run despite insufficient memory
     • Splits mem into pages
       • Pages are all fixed size
         • Can waste space
           • Is ineffeicent
       • Each job uses number of pages
         • Page tables are large and slow down computer
       • Index used to show where jobs are
       • To divide jobs is not a problem
   • Segmentation
     • Allows porgrams to run despite insufficient memory
     • Uses pages of variable size
       • Segments can be grouped logically, one segment per precedure
       • Reduces size of paging tables
       • More complicated
         • Need to store start address and size of segment
           • Start addresses need to be calculated with care
   • Virtual memory
     • Converts secondary mem to primary
       • Swaps pages inbetween them
         • This takes time
         • Causes disk threshing
           • Occurs when swapping pages, high rate of disk access and more time spent transferring then processing. You could just buy more ram or defragment the hard drive
       • Secondary mem is slower
     • Uses pages