Section 2: Systems Software

Question 1

What is an OS?

Answer 1

A program that manages computer operation. (Essentially a bridge between user and hardware).

Question 2

Where is the OS stored?

Answer 2

Permanent Storage (i.e. a HDD)

Question 3

What is a loader?

Answer 3

A loader is a small program (held in ROM) that sends instructions to copy the OS to RAM. This happens during boot-up.

Question 4

What are the main functions of an OS?

Answer 4

• Memory Management
• Interrupt Service Routines (ISR’s)
• Processor Scheduling
• Backing Store Management
• Management of all I/O.

Question 5

What is memory management?

Answer 5

Where the OS handles allocation of memory and management of space, using techniques such as paging and segmentation.

Question 6

What is paging?

Answer 6

• Memory is split into fixed page sizes (eg 4kb)

- A program may be held in non-contiguous pages.

Question 7

What is a page table?

Answer 7

A page table uses mapping to store a link between the physical memory address and the logical memory address.

Question 8

What is segmentation?

Answer 8

The logical division of address space into varying length segments.

Question 9

What is Virtual Memory?

Answer 9

Where the OS swaps pages to secondary storage.

Question 10

What is a disadvantage of Virtual Memory?

Answer 10

Processor time is consumed during the swap. If a computer is spending a large amount of time swapping files a performance drop will be noticeable since more time is spent swapping than on program tasks.

Question 11

What is an Interrupt?

Answer 11

A signal from a software program, hardware device or CPU internal clock.

INTERRUPTS ENABLE MULTI-TASKING!

Question 12

When does an Interrupt occur?

Answer 12

When an application program terminates or requires access to the OS services e.g. a printer out of paper.

It may also occur from a timer, to move the CPU onto the next process.

Question 13

What happens when the CPU receives an interrupt signal?

Answer 13

• It suspends execution of the current process and disables all lower priority interrupts.
• The values of the PC and other registers are put into a system stack, whilst the ISR is called to deal with the Interrupt.
• Dependant on the Interrupt type, a particular routine may be ran to service it.
• Once finished, the values are taken from the stack and put back into the CPU.

Question 14

What is special about Interrupts?

Answer 14

They have priorities. A low priority interrupt may be disabled if a higher priority is being serviced.

An example order is:

1) Power Fail Interrupt
2) Clock Interrupt
3) I/O device requests

Question 15

What is processor Scheduling?

Answer 15

The Where the OS queues tasks for max efficiency.

Question 16

What is multi-tasking?

Answer 16

Where the processor carries out small parts of each major task in turn, so it looks like it is carrying out several tasks simultaneously. This is because one processor can only carry out 1 task at a time.

Question 17

What is a Schedular?

Answer 17

Part of the OS responsible for ensuring processor time is used as efficiently as possible.

Question 18

What are the main requirements of the Schedular?

Answer 18

• Maximise Throughput
• Be fair to all multi-system users
• Provide acceptable response time
• Ensure hardware is kept as busy as possible.

Question 19

Explain the Round Robin Algorithm:

Answer 19

Each process is dispatched on a FIFO basis, each with a limited time slice (quantum).

The dispatcher gives the CPU the next process if the time expires or a high level Interrupt comes in.

An internal timer/interrupting clock generates this interrupt at specific times.

Question 20

What does Round Robin guarantee?

Answer 20

A reasonable response time to all users.

Question 21

Explain the First Come First Served Algorithm:

Answer 21

Jobs are processed in the order they come, with no system priorities.

Question 22

Explain the Shortest Remaining Time Algorithm:

Answer 22

Process with the shortest estimated time to complete run next.

This reduces the number of waiting jobs and the amount of small jobs behind big jobs.

Requires knowledge of how long a job will take, so job time needs to be estimated. Suitable for jobs such as payroll.

Question 23

Explain the Shortest Job First Algorithm:

Answer 23

Similar to shortest remaining time. For example, on a university campus, short programs will be ran quickly at peak usage times as opposed to large research projects.

Question 24

Explain the Multi Level Feedback Queue:

Answer 24

• Gives preference to short jobs.
• Gives preference to I/O bound processes.
• Separates process into categories based on their needs.

Several queues are implemented and jobs may move between them, dependent on processing time.

Question 25

Why must I/O device be kept as busy as possible?

Answer 25

I/O is slower than processor speed so by keeping it as busy as possible, bottlenecks don’t occur when multiple processes request access.

Question 26

Define Backing Store Management:

Answer 26

This is where the OS keeps a directory of where everything is stored, so it can be quickly accessed. It also knows which areas are free so new applications can be saved.

This is important because as a program is loaded, they are transferred from backing storage to memory.

Question 27

What is Peripheral Management:

Answer 27

Different applications require different I/O throughout their operation. A buffer is used to compensate for the difference between the I/O device and CPU.

Question 28

What are the different types of OS?

Answer 28

• Distributed OS

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