Chapter 20 System Software #2

Question 1

operating system

Answer 1

provides interface between users and hardware

–

Question 2

resources list

Answer 2

CPU
memory
I/O devices

–

Question 3

resource management

Answer 3

focuses on utilizing the resources and maximize their use
-deals with I/O operations

–

Question 4

direct memory access (DMA)

Answer 4

DMA controller is used to give access to memory directly. it allows the hardware to access the main memory independently of the CPU
frees up the CPU to allow it to carry out other tasks
DMA initiates data transfer while CPU carries out other tasks
once the data transfer is complete an interrupt signal is sent to the CPU from the DMA

–

Question 5

kernel

Answer 5

responsible for communication between hardware, software and memory
responsible for process, device and memory management

–

Question 6

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

Answer 6

provides interface e.g: GUI which helps to use the hardware
uses device drivers to synchronize the hardware

–

Question 7

difference between program and process

Answer 7

program is written code
process is executing code

–

Question 8

multitasking

Answer 8

to ensure multitasking operates correctly scheduling is used to decide which processes should be carried out
ensures the best use of computer resources by monitoring each state of process
kernel overlaps the execution of each process based on scheduling algorithms

–

Question 9

preemptive

Answer 9

when cpu is allocated to a particular process and if at that time a higher priority process comes then it is allocated to that process

–

Question 10

nonpreemptive

Answer 10

does not take any action until the process is terminated

–

Question 11

features of preemptive

Answer 11

resources are allocated to a process for a limited time
the process can be interrupted while it is running
more flexible form of scheduling

–

Question 12

features of nonpreemptive

Answer 12

once the resources are allocated to a process, the process retains them until it has completed its burst time
process cannot be interrupted while running
more rigid form of scheduling

–

Question 13

why does an operating system need to use scheduling algorithms

Answer 13

to allow multitasking to take place
to ensure fair usage of processer
to minimize the amount of time users must wait for their results
to keep cpu busy at all times
to ensure fair usage of memory
to ensure higher priority tasks are executed sooner

–

Question 14

ready state

Answer 14

process is not being executed
process is in the queue
waiting for the processor’s attention/time slice

–

Question 15

running state

Answer 15

process is being executed
process is currently using its allocated processor time/time slice

–

Question 16

blocked state

Answer 16

process is waiting for an event
so it cannot be executed at the moment
e.g: input/output

–

Question 17

ready -> running transmission conditions

Answer 17

current process no longer running // programmer is available
process was at the head of ready queue // process has highest priority
OS allocates processor to process so that process can execute

–

Question 18

running -> ready transmission conditions

Answer 18

when process is executing it is allocated a time slice
when time slice is completed, interrupt occurs and process can no longer use processor even though it is capable of further processing

–

Question 19

running -> blocked transmission conditions

Answer 19

process is executing when it needs to perform I/O operation and it is placed in blocked state until I/O operation is completed

–

Question 20

why is blocked -> running not possible

Answer 20

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

–

Question 21

why can a process not move directly from ready to blocked state

Answer 21

to be in blocked state, process must initiate I/O operation
to initiate operation process must be executing
if the process is in ready state it cannot be executing

–

Question 22

high level scheduler

Answer 22

decides which processes are to be loaded from backing store
into ready queue

–

Question 23

low level scheduler

Answer 23

decides which of the processes is in ready state
should get use of processor or which processor is put into running queue
based on position or priority

–

Question 24

first come first served scheduling

Answer 24

non-preemptive
based on arrival time
uses fifo principle

–

Question 25

shortest job first scheduling

Answer 25

non-preemptive
burst time of a process should be known in advance
processing requiring the least cpu time is executed first

–

Question 26

shortest remaining time first

Answer 26

preemptive
the processes are placed in ready queue as they arrive
but when a process with a shorter burst time arrives
existing process is removed
shorter process is then executed first

–

Question 27

round robin

Answer 27

preemptive
a fixed time slice is given to each process, this is known as time quantum
running queue is worked out by giving each process its time slice in the correct order (if a process completes before the end of its time slice then the next process is brought into ready queue for its time slice)

–

Question 28

interrupt

Answer 28

a signal to OS from the device

–

Question 29

round robin

Answer 29

preemptive

a fixed time slice is given to each process, this is known as time quantum

running queue is worked out by giving each process its time slice in the correct order (if a process completes before the end of its time slice then the next process is brought into ready queue for its time slice)

Question 30

interrupt

Answer 30

a signal to OS from the device which is connected to computer. sometimes interrupts are within the computer

processor will check for interrupt signals and will switch to kernel mode if any of the following signals are sent: device interrupt, exceptions, software interrupt

Question 31

IDT

Answer 31

interrupt dispatch table

to determine the current response to interrupts

Question 32

IPL

Answer 32

interrupt priority level

numbered (0-31)

Question 33

interrupt handling

Answer 33

when an interrupt is received, other interrupts are disabled so the process that deals with the interrupt cannot be itself be interrupted

state of current task/process is saved on the kernel stack

when the source of interrupt is identified, the priority of the interrupt is checked

system now jumps to the ISR

once completed, the state of the interrupted process is restored using the values stored on the kernel stack

after an interrupt has been handled, the interrupt needs to be restored so that any further interrupts can be dealt with

Question 34

page replacement

Answer 34

occurs when a requested page is not in memory

when paging in/out from memory, it is necessary to consider how the computer can decide which pages to replace to allow the requested page to be loaded

when a new page is requested but is not in memory, a page fault occurs

Question 35

optimal page replacement

Answer 35

looks forward in time to see which frame it can replace in the event of page fault

Question 36

longest resident

Answer 36

a particular page which is present for the longest time is swapped

Question 37

least used

Answer 37

a particular page which is used less is swapped

Question 38

internal fragmentation

Answer 38

when a process is allocated more memory than required few spaces are left in the block

Question 39

paging

Answer 39

a page is a fixed size block of memory

since the block size is fixed, it is possible that all blocks may not be fully used and this can lead to internal fragmentation

user provides a single value this means that the hardware decides the actual page size

procedures cannot be separated when using paging

Question 40

segmentation

Answer 40

a segment is a variable size block of memory

memory blocks are a variable size, this increases the risk of external fragmentation

the user will supply the segment number and segment size

procedures can be separated when using segmentation

Question 41

virtual memory

Answer 41

secondary storage used to extend the RAM

so cpu can access more memory space than available RAM

only part of program in use needs to be in RAM

data is swapped between RAM and disk

Question 42

how is paging used to manage virtual memory

Answer 42

divide memory RAM into frames

divide virtual memory into blocks of the same size called pages

frames/pages are a fixed size

set up a page table to translate logical to physical addresses

keep track of all free frames

swap pages in memory with new pages from disk when needed

Question 43

disk thrashing

Answer 43

pages are required back in RAM as soon as they are moved to disk

there is continuous swapping (at the same pages)

no useful processing happens

because pages that are in RAM and on disk are interdependent

nearly all processing time is used for swapping pages