Chapter 16.1

Question 1

what is an operating system

Answer 1

provides an interface between users and the hardware

Question 2

what is the purpose of an OS?

Answer 2

• interacts with hardware and runs software.
• provides multiprogramming ability
• maximises resources - CPU time, memory, I/O system. (manages process, device, and memory management)
• provides a user interface (CLI, GUI, voice recog.)
• provides file system to store data and programs
• manages peripherals

Question 3

resources list

Answer 3

cpu
memory
I/O devices

Question 4

maximise memory

Answer 4

moving frequently accessed instructions to cache for faster recall (SRAM over DRAM)
making use of virtual memory
partitioning memory (to allow for multiprogramming)

Question 5

kernel

Answer 5

program that controls all other programs. responsible for communication between hardware, software and memory.

Question 6

how does the operating system hide the complexities of the hardware from the user?

Answer 6

provides interface (CLI, GUI) - user doesnt interact directly with hardware
uses device drivers to synchronize hardware

Question 7

program

Answer 7

written/static code

Question 8

process

Answer 8

executing code

Question 9

how is multi-tasking achieved

Answer 9

scheduling is used to decide which processes should be carried out, which ensures the best use of computer resources (see scheduling)
kernel overlaps the execution of each process based on scheduling algorithms - CPU clock is also used!

Question 10

process states:

Answer 10

running = process is being executed
ready = process is in the queue waiting for the processor’s attention
blocked = process is waiting for an event so it cannot be currently executed, e.g. I/O

Question 11

why is blocked > running not possible?

Answer 11

when I/O operation is completed for process in the blocked state,
the process is transferred to the ready queue,
THEN the OS decides to allocate to processor

Question 12

preemptive scheduling algorithms

Answer 12

if a higher-priority process comes then the CPU’s attention is allocated to that process. examples are:
round robin and shortest time remaining

Question 13

non-preemptive scheduling algorithms

Answer 13

does not halt a process until it is completed or transitions to a waiting state. examples include:
first come first serve and shortest job first

Question 14

round robin

Answer 14

preemptive. each process gets a fixed time slice (all processes are given the same priority). when the time is up, if it’s still not completed the process is moved to the end of the ready queue.

benefit: starvation doesn’t occur, processes are given equal time slices
drawback: requires frequent switching, which can result in a high overhead

Question 15

first come first served

Answer 15

non-preemptive. each process is queued as it is received and is executed one by one.

benefit: starvation also doesn’t occur, every process eventually gets a chance to run.
drawback: is inefficient, less complex features

Question 16

shortest job first

Answer 16

non-preemptive. ready queue is sorted in ascending length of processes, shortest to longest.

benefit: more processes can be executed in a smaller amount of time, reduces average waiting time
drawback: prioritizes shorter processes, starvation is a possibility.
also doesn’t consider process priority

Question 17

shortest remaining time

Answer 17

preemptive. the processes are placed in ready queue as they arrive in ascending order but when a process with a shorter burst/remaining time arrives,
the running process is moved to the queue and the shorter process is set to run.

benefit: more processes get completed in a shorter time
drawback: starvation can occur

Question 18

purpose of scheduling

Answer 18

to allow multitasking
to ensure fair use of the processor, of the peripherals,
and of memory.
to ensure higher priority tasks are executed sooner
to prevent starvation of processes
to keep the CPU busy/efficient use of CPU
to minimise the user waiting time

Question 19

how does the kernel of the OS act as an interrupt handler?

Answer 19

other interrupts are disabled
state of current process is saved on the kernel stack
kernel calls the ISR (interrupt service routine) to deal with the interrupt.
interrupts are restored

Question 20

interrupt

Answer 20

a signal from some device indicating that an event has occurred, seeking the attention of the processor

Question 21

how is low level scheduling used to manage interrupt handling

Answer 21

An interrupt is of high priority therefore low-level scheduling will move the job straight into running, without going through the queue.

Question 22

low-level scheduling

Answer 22

decides which process in ready queue should be moved to running state, based on position and priority.
decides which program currently in memory should access the CPU.

Question 23

medium-level scheduling

Answer 23

occasionally takes a program back to the disk due to main memory being overcrowded

Question 24

high-level scheduling

Answer 24

makes decisions about which programs stored on the disk should be moved into memory to run and be executed

Question 25

paging

Answer 25

a page is a fixed size block of memory
since the block size is fixed, not all blocks may be fully used which can lead to internal fragmentation
OS decides the actual page size

Question 26

segmentation

Answer 26

segment is a variable size block of memory (increases the risk of external fragmentation)
divided by the programmer
segments are loaded when required during execution
each segment is loaded into a dynamic partition

Question 27

virtual memory

Answer 27

• secondary storage is used to extend the RAM/main memory,
• so the CPU can access more memory space than available RAM
• only the data in use needs to be in main memory, so data is swapped between RAM and virtual memory as needed
• virtual memory created is temporary

Question 28

the difference between paging and segmentation

Answer 28

paging divides memory into fixed size blocks, segmentation divides memory into variable size blocks

the OS decides page size, the user decides the segment size

access times for paging is faster than for segmentation

Question 29

disk thrashing

Answer 29

when pages are required in RAM as soon as they are moved to disk - pages in RAM and in virtual are interdependent
there is continuous swapping of the same pages, perpetual loading
deadlock occurs (no useful processing occurs)
processing time is used for swapping pages, processing speed is reduced

Question 30

how is paging used to manage virtual memory

Answer 30

divide RAM memory into frames
divide virtual memory into blocks of the same size called pages
frames and pages are a fixed size
set up a page table to translate logical (main) to physical (virtual) addresses, which keeps track of all free frames
swap pages in memory with new pages from disk when needed

Question 31

page replacement algorithms

Answer 31

first in first out = page most recently swapped is swapped back
least used = page which is used the least is swapped
longest resident = page which has been present for the longest time is swapped

Question 32

page

Answer 32

virtual memory (secondary storage) is divided into blocks of a fixed size

Question 33

page frame

Answer 33

the main memory is divided into page frames of the same size as the virtual memory pages

Question 34

page table

Answer 34

maps the pages to page frames and keeps track of all free frames