Operating Systems

Question 1

Operating Systems

Answer 1

Definition: Software that handles the interface to the hardware and manages resources.

Question 2

Features of OS

Answer 2

Provide and manage hardware resources. Provide an interface between the user and the machine e.g. CLI, GUI. Provide an interface between applications software and the machine e.g. saving to disk. Provide security for the data on the system e.g. passwords, access rights, encryption. Provide utility software to allow maintenance to be done e.g. housekeeping, anti-virus.

Question 3

Manage HW Resources

Answer 3

Management of memory – partitioning, allocating memory, virtual memory. Scheduling of Jobs including interrupt handling.

Question 4

Interrupt

Answer 4

An interrupt/signal sent to the processor. Request for processing time. Allows important tasks to be processed. Interrupts may cause a break in execution of the current routine so it can be resumed later.

Question 5

Reasons for interrupts

Answer 5

To obtain processor time…for a higher priority task. To avoid delays. To avoid loss of data. As an indicator to the processor… that a device needs to be serviced.

Question 6

Sources of interrupt

Answer 6

User e.g. new user logon request, power failure e.g. to allow orderly shut-down (prevent data loss), system failure, peripheral e.g. printer buffer empty, Clock, software e.g. divide by zero error.

Question 7

HW Interrupt

Answer 7

A hardware interrupt is an electronic alerting signal sent to the processor from an external device, either a part of the computer itself such as a disk controller or an external peripheral. E.g. imminent power failure.

Question 8

SW Interrupt

Answer 8

Software interrupt is an interrupt generated within a processor by executing an instruction. E.g. Divide by zero.

Question 9

Processing interrupts

Answer 9

After each cycle is completed (execute instruction) the processor checks the interrupt register by comparing priority of the current task with the highest priority interrupt. If interrupt is of higher priority the contents of registers put on stack …PC set to start address of interrupt service routine…interrupt is processed…check for further interrupts…contents of registers restored from stack and original task continues. If the interrupt was not of a higher priority than the current task then it starts the fetch-execute cycle again (the address of next instruction is copied from the PC to MAR).

Question 10

Resuming after interrupt

Answer 10

Reset interrupt request flag so shows interrupt has been serviced. Check interrupt register for further interrupts and service them if they are of higher priority than task to be resumed. If there are no higher priority interrupts then restore the contents of the registers from the stack and resume task.

Question 11

Interrupt priority

Answer 11

Interrupts have priorities: To decide between interrupt and current task. To choose which interrupt to process if 2 (or more) occur together. To ensure most urgent task is performed first. To ensure most efficient use of the processor. e.g. new user log on can wait… but data must be saved before power fails.

Question 12

Scheduling

Answer 12

The operating system allocates priorities to jobs. Deciding which job to run next is the job of the scheduler.

Question 13

Scheduler Purpose

Answer 13

Ensure all jobs are processed changing priorities where necessary. Process as many jobs as possible in the least possible time.
Maximise the number of interactive users receiving fast response times. Maximise throughput by utilising resources and processor time efficiently.

Question 14

Priority

Answer 14

An order of importance.

Question 15

Job States

Answer 15

Ready, Running and Blocked.

Question 16

Job Queue and Priorities

Answer 16

Jobs are given different priorities when in a job queue. This is because some jobs are more urgent than others. Priorities are also used to maximise the use of the computer resources. A job with a low priority can have its priority changed by the operating system.

Question 17

Job Leaving Running State 4 reasons

Answer 17

Job may have finished running. Job may require services of a peripheral (placed in the blocked queue to await servicing). Job has had long enough and it has to give up its place so that the other jobs get a chance. A higher priority interrupt comes along.

Question 18

Scheduling Algorithms

Answer 18

Round robin, system of priorities, length of job, first come, first served, multi-level feedback queues.

Question 19

Round robin

Answer 19

Each user is allocated a time slice. When time slice is up, system moves to next user. If next user needs processor, user given time slice. Repeat this for all users in turn (until all users serviced). Users may have different priorities which can change the order. Time slices are very small with no apparent delay for any user.

Question 20

Memory management

Answer 20

Physical and logical division of memory and programs (memory is not disk storage).

Question 21

Why mm is needed

Answer 21

To allocate memory to allow separate processes to run apparently simultaneously. To deal with allocation when paging/segmentation. To reallocate when necessary. To protect processes and data from each other. To protect the operating system and provide security. To enable memory to be shared.

Question 22

Virtual memory

Answer 22

Virtual memory combines active RAM and inactive memory on secondary storage to form a large range of contiguous addresses. Allows programs to run when there is insufficient memory available.

Question 23

Paging

Answer 23

Way of partitioning memory. Used for virtual memory. Segments stored on backing store and assigned to memory when needed. Pages are of a fixed size, they are made to fit sections of memory, they are physical divisions. Paged allocation divides the computer’s primary memory into fixed-size units called page frames, and the program’s address space into pages of the same size.

Question 24

Segmentation

Answer 24

Way of partitioning memory. Used for virtual memory. Segments stored on backing store and assigned to memory when needed. Segments are different sizes, complete sections of programs, logical divisions.

Question 25

Segment – Logical unit

Answer 25

Logical unit such as: main program , procedure, function, method, object, local variables, global variables, stack, arrays.

Question 26

Disk threshing – problem with VM

Answer 26

Occurs when using virtual memory moving pages between memory & disk. Disk is relatively slow. High rate of disk access more time spent swapping pages than on processing - computer may ‘hang’.

Question 27

Spooling

Answer 27

Simultaneous Peripheral Operations On-Line. It is a type of buffering. It is used to place input and output on a fast access storage device, such as a disk, so that slow peripheral devices do not hold up the processor.

Question 28

Reasons for spooling

Answer 28

It allows sharing of the printer on a network with different users. It avoids delays and frees up the processor. It avoids the speed mismatch between the slow printer and fast processor. It allows jobs to be prioritised.

Question 29

How Print Spooling works

Answer 29

Output data to disk drive/storage device for printing at another time. To allow sharing on a network. Job references stored in a queue. Avoids speed mismatch as printers are relatively slow. Jobs can be prioritised. Print Spoolers can notify users when their output has been printed, distribute jobs among several printers and generate banner pages to identify and separate print jobs.

Question 30

Other use of spooling

Answer 30

A batch processing system uses spooling to maintain a queue of ready-to-run jobs which can be started as soon as the system has the resources to process them.

Question 31

Boot file

Answer 31

A boot file is a computer program executed automatically that loads the main operating system for the computer after completion of the self-tests – POST. It is stored on ROM.

Question 32

Purpose of Boot File

Answer 32

Provides personal settings, initialises operating system, passes control to operating system (by setting program counter), checks hardware, checks memory.

Question 33

Booting up

Answer 33

The initial set of operations performed after CPU receives power or when the computer is reset. The first step is for the computer to run the Power-On-Self-Test (POST) routine resident in permanent memory. The boot program is stored in ROM and contains the skeleton of the Basic Input/Output System. Control is now passed to the boot program. The boot program instructs the CPU to send signals to check hardware (buses, system clock, memory) and peripherals. The boot program takes account of any personal settings e.g. which operating system to boot. The boot program now reads the operating system into memory and then executes it (by setting Program Counter).

Question 34

Duties of BIOS during POST

Answer 34

Verify CPU registers. Find, size, and verify system main memory. Initialize BIOS. Identify, organize, and select which devices are available for booting. Provide a user interface for system’s configuration. BEEP codes to indicate errors.

Question 35

FAT

Answer 35

File Allocation Table - a map of where files are stored on the hard disk.

Question 36

FAT – use

Answer 36

Is updated by the operating system when files are saved/deleted. Is used by the operating system when files are accessed.

Question 37

FAT – contents

Answer 37

Provides addresses/pointers to the start of files. Provides pointers to further clusters (in a chain). Stores file names
Stores file sizes. Stores access rights. Identifies free space.