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A Level CIE CS 9618 Paper 3 > U16 system software > Flashcards

U16 system software Flashcards

purpose of an operating system

1
Q

operating system

A

provides interface between users and hardware

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2
Q

resources list

A
  • CPU
  • memory
  • I/O devices
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3
Q

resource management

A
  • focuses on utilizing the resources and maximize their use
    -deals with I/O operations
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4
Q

direct memory access (DMA)

A
  • 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
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5
Q

kernel

A
  • responsible for communication between hardware, software and memory
  • responsible for process, device and memory management
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6
Q

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

A
  • provides interface e.g: GUI which helps to use the hardware
  • uses device drivers to synchronize the hardware
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7
Q

difference between program and process

A
  • program is written code
  • process is executing code
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8
Q

multitasking

A
  • 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
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9
Q

preemptive

A

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

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10
Q

nonpreemptive

A

does not take any action until the process is terminated

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11
Q

features of preemptive

A
  • resources are allocated to a process for a limited time
  • the process can be interrupted while it is running
  • more flexible form of scheduling
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12
Q

features of nonpreemptive

A
  • 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
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13
Q

why does an operating system need to use scheduling algorithms

A
  • 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
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14
Q

ready state

A
  • process is not being executed
  • process is in the queue
  • waiting for the processor’s attention/time slice
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15
Q

running state

A
  • process is being executed
  • process is currently using its allocated processor time/time slice
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16
Q

blocked state

A
  • process is waiting for an event
  • so it cannot be executed at the moment
  • e.g: input/output
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17
Q

ready -> running transmission conditions

A
  • 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
18
Q

running -> ready transmission conditions

A
  • 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
19
Q

running -> blocked transmission conditions

A
  • process is executing when it needs to perform I/O operation and it is placed in blocked state until I/O operation is completed
20
Q

why is blocked -> running not possible

A
  • when I/O operation completed for process in blocked state
  • process is transferred to ready state
  • OS decides to allocate to processor
21
Q

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

A
  • 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
22
Q

high level scheduler

A
  • decides which processes are to be loaded from backing store
  • into ready queue
23
Q

low level scheduler

A
  • 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
24
Q

first come first served scheduling

A
  • non-preemptive
  • based on arrival time
  • uses fifo principle
25
Q

shortest job first scheduling

A
  • non-preemptive
  • burst time of a process should be known in advance
  • processing requiring the least cpu time is executed first
26
Q

shortest remaining time first

A
  • 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
27
Q

round robin

A
  • 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)
28
Q

interrupt

A
  • 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
29
Q

IDT

A
  • interrupt dispatch table
  • to determine the current response to interrupts
30
Q

IPL

A
  • interrupt priority level
  • numbered (0-31)
31
Q

interrupt handling

A
  • 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
32
Q

page replacement

A
  • 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
33
Q

optimal page replacement

A

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

34
Q

longest resident

A

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

35
Q

least used

A

a particular page which is used less is swapped

36
Q

internal fragmentation

A

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

37
Q

paging

A
  • 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
38
Q

segmentation

A
  • 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
39
Q

virtual memory

A
  • 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
40
Q

how is paging used to manage virtual memory

A
  • 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
41
Q

disk thrashing

A
  • 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

A Level CIE CS 9618 Paper 3 (6 decks)

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