Chapter 7 - Functions of an operating system

Question 1

OS

Answer 1

• manages the hardware and provides an interface for the user and the application software
• boot loader in ROM loads OS into RAM when power is on

Question 2

Functions - Memory management

Answer 2

• Manages allocation of RAM to different programs (or virtual RAM if RAM fills up)
• benefits - better security (doesn’t let programs access memory reserved for other programs); multitasking (allows multiple programs to run at once)

Question 3

Memory management - Paging and segmentation

Answer 3

Paging:
- available memory divided into fixed-size chunks (4KB) with an address
- each program is allocated sufficient pages, but they might not always be next to each other (contiguous), which slows down access
- page table uses mapping to store link between physical memory address and logical address spaces

(or) Segmentation:
- Memory is divided into segments of different lengths, so programs will be allocated the exact length they need

Question 4

Functions - Interrupts

Answer 4

• Can be sent to CPU by software, hardware or clock to carry out a more important task
• CPU checks at the end of each clock cycle whether there are interrupts to be processed
• State of CPU needs to be stored to resume the cycle once interrupt is taken care of:
  1. Processor stops fetching instructions
  2. Pushes current contents of registers onto a stack (a data structure with last in, first out order)

Interrupt priority – if another interrupt with a higher priority comes up during an interrupt, the state of the registers during the previous interrupt are pushed onto the stack and the newer interrupt is taken care of

Question 5

Functions - Processor scheduling

Answer 5

• OS schedules when each app can use the CPU, giving the illusion of multi-tasking (multiple apps running at the same time)
• Aims of scheduling: Provide an acceptable response time to all tasks; Maximise CPU time when fully engaged; Ensure fairness in allocating time to each task

Question 6

Scheduling algorithms - Round robin

Answer 6

FIFO (first in first out), each job allocated time slice for CPU time, if job is not complete by the end of its time slice, next job is allocated a time slice

Question 7

Scheduling algorithms - First come first served

Answer 7

First job to arrive is executed until it is completed, unless if something has a higher priority

Question 8

Scheduling algorithms - Shortest time remaining

Answer 8

Give priority to task which closest to finishing (estimated when task arrives), problem is that a long task might be starved of CPU time

Question 9

Scheduling algorithms - Shortest job first

Answer 9

Total execution time estimated and shortest execution time job completed first because other tasks are waiting for less time

Question 10

Scheduling algorithms - Multi-level feedback queues

Answer 10

• aim: maximise processor use, so gives preference to short jobs; gives preference to I/O bound processes; separates processes into categories based on processor time
• jobs move between queues depending on how much processor time they use
• I/O processes are slow so they are kept running all the time to prevent bottlenecks

Question 11

Functions - Backing store management

Answer 11

• files/apps are transferred from backing storage into memory when loaded
• file management system: OS keeps track of where files are stored, which areas of storage are free so they can be easily accessed/allocated space in RAM

Question 12

Functions - Peripheral management

Answer 12

Printing:
1. OS checks if printer is switched on and online
2. checks if it is a printer (and not a keyboard for e.g.)
3. transfers data to be printed
4. data held at buffer as it waits to be printed