Chapter 7 - Functions of an operating system

Chapter 7 - Functions of an operating system

1. Memory Management

   • A PC allows users to work on several tasks at once
   • Each program is allocated a specific area in memory while running
   • Allocation and management of space is controlled by the operating system

2. Processor Scheduling

   • Round Robin
     • Processes despatched on a first in first out basis
     • Each process is given a limited amount of CPU time (time slice)
     • If process is not complete before times up, the despatcher gives CPU to next process
     • Operating system sets an interval timer to generate interrupts at specific times
     • This method guarantees a resonable response time to all users
   • First Come First Served
     • Jobs processed in the order they arrive in
   • Shortest remaining time
     • Normally reduces number of waiting jobs
     • Requires knowledge of how long a job will take
     • Process with smallest estimated time to completion run next
   • Shortest Job First
     • Process with smallest estimated running time is run next
     • advantages and constraints same as 'shortest remaining time
   • Multi-level feedback queues
     • Designed to give preference to short jobs and I/O bound processes
     • Separates process into categories based on need
     • Implements several job queues
       • Jobs can move between queues depending on processor time

3. Interrupts

   • A signal from a software program, hardware device or internal clock to the CPU
   • Occurs when an application program terminates/requests certain services
   • Interrupt Service Routines
     • When receiving an interrupt, the CPU suspends execution of the running program/process
       • It also disables interrupts of lower priority
     • It puts the values of the program counter and each register onto the system stack
     • An interrupt service routine is called to deal with the interrupt
     • Interrupts are assigned priorities. There are several types which are given certain priorities
       • An I/O device sends a signal requesting service or signalling end of I/O operations
       • Power-fail interrupt
       • Clock interrupt

4. Backing store managment

   • When apps are loaded, they are transferred from backing storage to memory
   • The operating system needs to keep a directory of where files are stored
   • The operating system also needs to know where free storage is available
   • The file management system enables users to:
     • Move files/folders
     • delete files
     • protect others from unauthorized access

5. Peripheral Managment

   • applications require different input/output devices
   • When printing a file, the operating system communicates with the printer
   • Data to be sent to output devices sent to an area of memory called a buffer
   • The buffer compensates the difference in speed between the CPU and output devoces

6. Paging and Segmentation

   • Paging
     • Memory is divided into fixed pages
     • Page tables links a physical memory address and a logical memory address
     • Data may be stored across multiple pages
   • Segmentation
     • Logical division of address space into varying segments
     • Possible to load only part of a program into memory initially

7. Virtual Memory

   • Memory is not limitless so your PC will need extra memory
   • Virtual memory is a part of secondary storage that acts as main meory