16.1 OS Purposes (A Level)
A Level · 18 questions found
What this topic covers
Section titled “What this topic covers”- How OS maximises resource use; UI hides hardware complexity from users
- Multi-tasking and process states: running, ready, blocked
- Scheduling algorithms: round robin, shortest job first, FCFS, shortest remaining time
- Kernel as interrupt handler; interrupt handling in low-level scheduling
- Virtual memory, paging and segmentation; page replacement; disk thrashing
Past paper questions
Section titled “Past paper questions”(a) A scheduling routine determines how processes are managed by the operating system. 2 marks
Identify two scheduling routines.
1
2
(b) Describe two ways in which the complexities of the computer hardware are hidden from the user. 4 marks
1
2
Show mark scheme
4(a) [2 marks]
One mark per scheduling routine ( Max 2 ) from: • Round robin • Shortest job first • First come first served • Shortest remaining time
4(b) [4 marks]
One mark for identification and one mark for a description ( Max 4 ) Two from: Provision of a User Interface // Provision of a Graphical User Interface [1] Allows the user to interact with the computer in a more intuitive way // Icons and menus are used to control devices by simply ‘pointing and clicking’ [1] Use of device drivers [1] Makes it easier to control peripherals such as printers within the operating system of the computer rather than on the separate device itself [1] Device mapping [1] Different devices (physical and virtual) are easy to identify on the network, check their status, or use [1] The user interacts only with the Application / top layer (of the TCP/IP protocol suite) [1] leaving the lower layers and their complexities hidden from the user [1]
(a) Describe how interrupt handling is used in low-level scheduling. 2 marks
(b) In process management, a process can be in one of three process states: running, ready or blocked. 3 marks
Complete the table to identify one reason why a process could be in each of the three states.
| Process state | Reason |
|---|---|
| running | |
| ready | |
| blocked |
Show mark scheme
5(a) [2 marks]
One mark per mark point ( Max 2 ) MP1 The low-level scheduler manages the handling of interrupts based on priority MP2 … ensuring that critical events are handled without delay MP3 It uses an Interrupt Vector Table (IVT) / Interrupt Descriptor/Despatch Table (IDT) / Interrupt Service Routine (ISR) lookup MP4 … to map the interrupt to the specific handling routine / ISR.
5(b) [3 marks]
One mark per correct answer ( Max 3 ) Process state Reason running CPU time has been allocated and the process is being executed. The process is waiting (in the ready queue) for a slice of CPU time. It could run. // It has been displaced ready by a higher priority process and could otherwise still run. blocked The process is waiting for an I/O operation / some event to take place / be completed.
•
(a) State the purpose of multi‑tasking. 1 mark
(b) Explain how an operating system ensures that multi‑tasking operates efficiently. 3 marks
Show mark scheme
5(a) [1 mark]
To enable multiple programs/processes to be executed at the same time
5(b) [3 marks]
One mark per mark point ( Max 3 ) MP1 the operating system monitors the state of each task/process MP2 using scheduling to ensure hardware resources are used efficiently MP3 … and making sure that tasks/processes do not clash.
(a) Describe what is meant by multi-tasking and how it benefits process management. 2 marks
(b) Explain the function of the shortest remaining time scheduling routine and give a benefit of this routine. 4 marks
Function
Benefit
Show mark scheme
8(a) [2 marks]
One mark for each mark point MP1 Running multiple processes concurrently MP2 … which benefits process management by allowing more tasks to complete than would be the case if they had to run one task after another.
8(b) [4 marks]
One mark for each mark point ( Max 3 ) MP1 The processes are queued as they arrive MP2 Processes with the shortest burst time are executed first MP3 It is a pre-emptive scheduling function // When a process with a shorter burst time arrives the existing process is replaced by the shorter process. MP4 The scheduler will continue to choose shorter processes over longer processes if they continue to be added to the queue can cause starvation for longer jobs One mark for benefit ( Max 1 ) e.g. MP5 Processes with a short burst time are processed very quickly MP6 Waiting time is minimised
The management and scheduling of processes are tasks carried out by an operating system.
(a) Identify three process states. 3 marks
1
2
3
(b) Describe the function of the shortest job first scheduling routine and give a benefit of this routine. 4 marks
Function
Benefit
Show mark scheme
5(a) [3 marks]
One mark per point • Running state • Ready state • Blocked state
5(b) [4 marks]
One mark per mark point ( Max 3 ) MP1 The processes are queued as they arrive MP2 The process with the shortest time to complete/burst time is selected first and executed MP3 The process will continue until complete or put in a waiting state once execution has begun // it is non-pre-emptive MP4 The scheduler will continue to choose shorter processes over longer processes if they continue to be added to the queue can cause starvation for longer jobs One mark for a benefit ( Max 1 ) MP5 Shorter jobs don’t have to wait for longer jobs to complete before processing // Significantly reduces the average overall waiting time for processes // Ensures starvation doesn’t occur for process with shorter burst times MP6 Higher throughput of processes
The management and scheduling of processes are tasks carried out by an operating system.
(a) Describe one reason why scheduling is necessary in process management. 2 marks
(b) Explain the function of the round robin scheduling routine and give a benefit of this routine. 4 marks
Function
Benefit
Show mark scheme
6(a) [2 marks]
One mark for each mark point ( Max 2 ) • Process scheduling is required to ensure that all processes are executed in a timely manner • … and enables multitasking/multiprogramming/multiprocessing • … to minimise CPU idle time • … to ensure that no process is starved of resources. // … to ensure fair access to resources. • … ensures jobs/processes are completed in order of priority.
6(b) [4 marks]
One mark for each mark point ( Max 3 ) MP1 Processes are queued as they arrive. MP2 It is a pre-emptive scheduling routine. MP3 A fixed time quantum is given to each process. // Each process has an equal time slice. MP4 When a time slice ends, the status of the process is saved/queued so it can continue from where it left off in its next time slice. MP5 and the next process is executed for its time slice; its previous state is reinstated/restored, if applicable. MP6 If a process completes within its time slice, the next process is executed for its time slice. One mark for benefit ( Max 1 ) MP7 Reduces average response time by limiting each process to a fixed amount of time. MP8 No issue with starvation of resources.
(a) Describe the process of segmentation for memory management. 4 marks
(b) Explain what is meant by disk thrashing. 3 marks
Show mark scheme
8(a) [4 marks]
One mark per mark point ( Max 4 ) MP1 In segmented memory, the logical / virtual address space is broken into varying sized blocks called segments / sections. MP2 Each segment has a name and size. MP3 During execution segments from logical / virtual memory are loaded into physical memory. MP4 The address is specified by the user MP5 … it contains the segment name and offset value. MP6 Segments are numbered MP7 … and this number is used as an index in the segment map table. MP8 The offset value determines the size of the segment. MP9 A segment map table maps logical / virtual addresses to physical addresses / contains the segment number and offset.
8(b) [3 marks]
One mark per mark point ( Max 3 ) MP1 Disk thrashing is a problem that may occur when virtual memory is being used. MP2 As the main memory fills up, more and more pages need to be swapped in and out of virtual memory. MP3 This swapping leads to a very high rate of hard disk access / excessive disk head movements. MP4 Moving a hard disk read/write head takes a relatively long time / long latency time. MP5 Eventually, more time is spent swapping pages than processing data thrash point, which can cause the program to freeze or not run.
(a) The kernel is the central component of an Operating System (OS). 2 marks
Outline how the kernel of an OS acts as an interrupt handler.
(b) (i) State what is meant by the term multi‑tasking in an Operating System. 1 mark
(ii) Describe how multi‑tasking is implemented in an Operating System. 2 marks
Show mark scheme
9(a) [2 marks]
One mark per mark point ( Max 2) MP1 the kernel receives a signal when an interrupt is generated MP2 the kernel checks the priority and reviews the status/priority of the current interrupts MP3 system enters kernel mode if the type of interrupt is of higher priority than the current process MP4 the kernel consults the interrupt dispatch table / IDT MP5 … and saves the state of the interrupted process / contents of the registers on the kernel stack MP6 the kernel restores the process state e.g. contents of registers once the interrupt is serviced
9(b)(i) [1 mark]
One mark per mark point ( Max 1) MP1 multi-tasking allows computers to carry out / seem to carry out more than one process at a time
9(b)(ii) [2 marks]
One mark per mark point ( Max 2) MP1 processor time/common hardware and resources is/are shared between tasks MP2 scheduling is used to decide on the processes to be carried out to ensure multi-tasking operates correctly / efficiently / without clashes MP3 one task of a higher priority can interrupt another task that is currently running
(a) Describe the process of segmentation for memory management. 4 marks
(b) Explain what is meant by disk thrashing. 3 marks
Show mark scheme
8(a) [4 marks]
One mark per mark point ( Max 4 ) MP1 In segmented memory, the logical / virtual address space is broken into varying sized blocks called segments / sections. MP2 Each segment has a name and size. MP3 During execution segments from logical / virtual memory are loaded into physical memory. MP4 The address is specified by the user MP5 … it contains the segment name and offset value. MP6 Segments are numbered MP7 … and this number is used as an index in the segment map table. MP8 The offset value determines the size of the segment. MP9 A segment map table maps logical / virtual addresses to physical addresses / contains the segment number and offset.
8(b) [3 marks]
One mark per mark point ( Max 3 ) MP1 Disk thrashing is a problem that may occur when virtual memory is being used. MP2 As the main memory fills up, more and more pages need to be swapped in and out of virtual memory. MP3 This swapping leads to a very high rate of hard disk access / excessive disk head movements. MP4 Moving a hard disk read/write head takes a relatively long time / long latency time. MP5 Eventually, more time is spent swapping pages than processing data thrash point, which can cause the program to freeze or not run.
(a) A student buys a new computer. 2 marks
State one benefit to the student of a user interface and give an example.
Benefit
Example
(b) Two of the process states are the running state and the ready state. 1 mark
Identify one other process state.
(c) Outline conditions under which a process could change from the running state to the ready state. 2 marks
Show mark scheme
7(a) [2 marks]
One mark for a benefit ( Max 1 ) e.g. MP1 The user interface hides the complexities of the computer hardware/operating system from the user MP2 It provides appropriate access systems for users with differing needs MP3 Complex commands involving memory locations/buses/computer hardware/ are avoided One mark for a valid example ( Max 1 ) e.g. Clicking on icon rather than writing code Using a graphical user interface / icons for navigation
7(b) [1 mark]
Blocked (state)
7(c) [2 marks]
One mark per mark point ( Max 2 ) MP1 When the time slice of the running process expires (round robin). MP2 …and there is a process with a higher priority in the ready queue, the running process is pre-empted MP3 When an interrupt arrives at the CPU, (the process running on the CPU gets pre-empted).
Show mark scheme
8 [5 marks]
One mark per mark point - working ( Max 3 ) May be seen on diagram or in working section MP1 Initialisation – setting Start to 0 MP2 …and the rest of the towns to MP3 Evidence to show values at nodes being updated MP4 Evidence to show ‘visited node(s)’ MP5 Evidence to show a correct calculation of at least one route MP6 Evidence to show more than one route has been calculated for at least one town Correct Answers ( Max 2 ) Two marks for all six correct values One mark for four or five correct values. 8 12 19 14 16 10
(a) A student buys a new computer. 2 marks
State one benefit to the student of a user interface and give an example.
Benefit
Example
(b) Two of the process states are the running state and the ready state. 1 mark
Identify one other process state.
(c) Outline conditions under which a process could change from the running state to the ready state. 2 marks
Show mark scheme
7(a) [2 marks]
One mark for a benefit ( Max 1 ) e.g. MP1 The user interface hides the complexities of the computer hardware/operating system from the user MP2 It provides appropriate access systems for users with differing needs MP3 Complex commands involving memory locations/buses/computer hardware/ are avoided One mark for a valid example ( Max 1 ) e.g. Clicking on icon rather than writing code Using a graphical user interface / icons for navigation
7(b) [1 mark]
Blocked (state)
7(c) [2 marks]
One mark per mark point ( Max 2 ) MP1 When the time slice of the running process expires (round robin). MP2 …and there is a process with a higher priority in the ready queue, the running process is pre-empted MP3 When an interrupt arrives at the CPU, (the process running on the CPU gets pre-empted).
Show mark scheme
9(a) [2 marks]
One mark for each correct point ( Max 2 ) • Uses artificial neural network(s) • … that contain(s) a high number of hidden layers • … modelled on the human brain. • Deep learning uses many layers to progressively extract higher level features from the (raw) input. • Deep learning is a specialised form of machine learning.
9(b) [2 marks]
One mark for each correct point ( Max 2 ) • Deep learning makes good use of unstructured data. • Deep learning outperforms other methods if the data size is large. • Deep learning systems enable machines to process data with a nonlinear approach. • Deep learning is effective at identifying (hidden) patterns / patterns that humans might not be able to see / patterns that are too complex / time consuming for humans to carry out. • It can provide a more accurate outcome with higher numbers of hidden layers.
10 (a) Define these Object-Oriented Programming (OOP) terms:
Encapsulation
Getter
Setter 3 marks
(b) A school has a program written using OOP to maintain its staff and student records. 3 marks
The object SubstituteTeacher allows the details of the school’s substitute teachers to be
stored. This includes their full name, telephone number and whether or not they are in school
today. For example:
| SubName | Sarah Jones |
|---|---|
Telephone |
01223658721 |
InSchool |
TRUE |
Complete the diagram for the object SubstituteTeacher, including appropriate properties
and their getters and setters.
SubstituteTeacher
SubName : STRING
InSchool : BOOLEAN
SetTelephone(Tel : STRING)
GetSubName()
Show mark scheme
9(a) [3 marks]
One mark for each point ( Max 3 ) • Process scheduling allows more than one program/task to appear to be executed at the same time / enables multi- tasking / multiprogramming. • To allow high priority jobs to be completed first. • To keep the CPU busy all the time • … to ensure that all processes execute efficiently • … and to have reduced wait times for all processes / to ensure all processes have fair access to the CPU / prevent starvation of some processes.
9(b) [6 marks]
One mark for each point ( Max 2 ) for Shortest job first : • Process are executed in ascending order of the amount of CPU time required // Short processes are executed first followed by longer processes. • …which leads to an increased throughput (because more processes can be executed in a smaller amount of time). One mark for each point ( Max 2 ) for Round robin : • Each process is served by the CPU for a fixed time/time slice (so all processes are given the same priority). • Starvation doesn’t occur (because for each round robin cycle, every process is given a fixed time/time slice to execute). One mark for each point ( Max 2 ) for First come first served : • No complex logic, each process request is queued as it is received and executed one by one. • Starvation doesn’t occur (because every process will eventually get a chance to run) // less processor overhead.
Show mark scheme
9(a) [2 marks]
One mark for each correct point ( Max 2 ) • Uses artificial neural network(s) • … that contain(s) a high number of hidden layers • … modelled on the human brain. • Deep learning uses many layers to progressively extract higher level features from the (raw) input. • Deep learning is a specialised form of machine learning.
9(b) [2 marks]
One mark for each correct point ( Max 2 ) • Deep learning makes good use of unstructured data. • Deep learning outperforms other methods if the data size is large. • Deep learning systems enable machines to process data with a nonlinear approach. • Deep learning is effective at identifying (hidden) patterns / patterns that humans might not be able to see / patterns that are too complex / time consuming for humans to carry out. • It can provide a more accurate outcome with higher numbers of hidden layers.
(a) Draw one line to connect each Operating System (OS) term to the most appropriate description about it. 5 marks
OS term Description
Multi-tasking
Paging
Interrupt handling
Scheduling
Virtual memory
Using secondary storage to simulate additional main memory
Managing the processes running on the CPU
Managing the execution of many programs that appear to run at the same time
Locating non-contiguous blocks of data and relocating them
Transferring control to another routine when a service is required
Reading/writing same-size blocks of data from/to secondary storage when required
(b) Explain how an interpreter executes a program without producing a complete translated version of it. 4 marks
Show mark scheme
3(a) [5 marks]
One mark for each correct line from Operating System Term to Description Managing the execution of many programs time that appear to run at the same
3(b) [4 marks]
One mark for each correct statement (Max 4) An interpreter examines source code one statement at a time • Check each statement for errors • …If no error is found the statement is executed • …If an error is found this is reported and the interpreter halts • Interpretation is repeated for every iteration in repeated sections of • code/in loops Interpretation has to be repeated every time the program is run •
(a) Draw one line to connect each Operating System (OS) term to the most appropriate description about it. 5 marks
OS term Description
Multi-tasking
Paging
Interrupt handling
Scheduling
Virtual memory
Using secondary storage to simulate additional main memory
Managing the processes running on the CPU
Managing the execution of many programs that appear to run at the same time
Locating non-contiguous blocks of data and relocating them
Transferring control to another routine when a service is required
Reading/writing same-size blocks of data from/to secondary storage when required
(b) Explain how an interpreter executes a program without producing a complete translated version of it. 4 marks
Show mark scheme
3(a) [5 marks]
One mark for each correct line from Operating System Term to Description Managing the execution of many programs time that appear to run at the same
3(b) [4 marks]
One mark for each correct statement (Max 4) An interpreter examines source code one statement at a time • Check each statement for errors • …If no error is found the statement is executed • …If an error is found this is reported and the interpreter halts • Interpretation is repeated for every iteration in repeated sections of • code/in loops Interpretation has to be repeated every time the program is run •
(a) Draw one line to connect each Operating System (OS) term to the most appropriate description about it. 5 marks
OS term Description
Multi-tasking
Paging
Interrupt handling
Scheduling
Virtual memory
Using secondary storage to simulate additional main memory
Managing the processes running on the CPU
Managing the execution of many programs that appear to run at the same time
Locating non-contiguous blocks of data and relocating them
Transferring control to another routine when a service is required
Reading/writing same-size blocks of data from/to secondary storage when required
(b) Explain how an interpreter executes a program without producing a complete translated version of it. 4 marks
Show mark scheme
3(a) [5 marks]
One mark for each correct line from Operating System Term to Description Managing the execution of many programs time that appear to run at the same
3(b) [4 marks]
One mark for each correct statement (Max 4) An interpreter examines source code one statement at a time • Check each statement for errors • …If no error is found the statement is executed • …If an error is found this is reported and the interpreter halts • Interpretation is repeated for every iteration in repeated sections of • code/in loops Interpretation has to be repeated every time the program is run •