System Software - Worksheets, Questions and Revision

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GCSE · Computer Systems

P1.4 System Software

OCR J277 · Calculators not allowed · about 90 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

State two functions that are carried out by the operating system of a computer.

(Total for Question 1 is 2 marks)

Which one of these is NOT a standard function of an operating system?

A) Memory management
B) Peripheral management
C) Compiling source code into machine code
D) User management

(Total for Question 2 is 1 mark)

Complete the table below by naming the type of utility software described in each row.

Description	Type of utility software
1) Reorganises files stored on a hard disk drive so that fragmented parts of a file are stored closer together, reducing the time taken to access them.	?
2) Scrambles data using an algorithm and a key so that it cannot be understood if it is intercepted.	?
3) Reduces the size of a file so that it takes up less storage space or can be transferred more quickly.	?

(Total for Question 3 is 3 marks)

A student connects a new 3D printer to their laptop for the first time. Describe how a device driver allows the operating system to communicate with the new printer.

(Total for Question 4 is 3 marks)

Explain what is meant by an interrupt, and explain why interrupts are assigned different priority levels.

(Total for Question 5 is 3 marks)

Describe, using an example, how memory management by the operating system allows a laptop to run more than one application at the same time.

(Total for Question 6 is 4 marks)

Modern operating systems use virtual memory to manage RAM.

(a)State what is meant by the term 'virtual memory'.(2)

(b)A student notices their laptop has become extremely slow after opening many large applications at once. Explain, in terms of virtual memory, why this has happened.(3)

(Total for Question 7 is 5 marks)

Explain the difference between lossy and lossless compression, using an example of when each would be an appropriate choice.

(Total for Question 8 is 4 marks)

A simple black and white image is stored as a single row of 12 pixels, shown below, where W = white and B = black:

WWWWWBBBWWWW

(a)Use run-length encoding (RLE) to encode this row of pixels. Show your encoding as pairs of (colour, run length).(2)

(b)Each (colour, run length) pair is stored using 2 bytes: 1 byte for the colour and 1 byte for the run length. Calculate the compression ratio achieved, given that storing the original row (uncompressed, 1 byte per pixel) uses 12 bytes. Give your answer in the form x:1.(2)

(c)Explain why run-length encoding is an effective compression technique for this image but would be far less effective for a photograph of a busy street.(2)

(Total for Question 9 is 6 marks)

The pseudocode below, written in the OCR Exam Reference Language, simulates round robin scheduling of three tasks sharing a processor, with a fixed time slice of 2:

tasks <- [6, 3, 5]
timeSlice <- 2
round <- 0
WHILE tasks[0] > 0 OR tasks[1] > 0 OR tasks[2] > 0
  round <- round + 1
  FOR i <- 0 TO 2
    IF tasks[i] > 0 THEN
      IF tasks[i] >= timeSlice THEN
        tasks[i] <- tasks[i] - timeSlice
      ELSE
        tasks[i] <- 0
      ENDIF
    ENDIF
  NEXT i
  OUTPUT round, tasks[0], tasks[1], tasks[2]
ENDWHILE

(a)Complete the trace table below by tracing this algorithm.

Round	tasks[0]	tasks[1]	tasks[2]
1	?	?	?
2	?	?	?
3	?	?	?

(3)

(b)State after how many rounds all three tasks have finished, and explain how you can tell this from the WHILE loop's condition.(3)

(Total for Question 10 is 6 marks)

Symmetric encryption uses the same key to encrypt and decrypt data. Describe how symmetric encryption works, and explain one problem this creates for sending encrypted data over the internet.

(Total for Question 11 is 3 marks)

Write an algorithm, using the OCR Exam Reference Language, that performs run-length encoding on a string of characters. Your algorithm should: input a string from the user; identify each run of identical consecutive characters; output each run as the character followed immediately by the length of the run (for example, the string 'aaabb' should output 'a3b2').

(Total for Question 12 is 6 marks)

Solid state drives (SSDs) do not benefit from being defragmented, whereas traditional hard disk drives (HDDs) do. Explain why defragmentation improves performance on an HDD, and explain why it is not needed (and can even reduce lifespan) on an SSD.

(Total for Question 13 is 4 marks)

A memory address is given in hexadecimal as 2C.

(a)Convert this address to denary.(1)

(b)Show that 44 in denary converts to 00101100 in 8-bit binary.(2)

(Total for Question 14 is 3 marks)

A small UK photography business wants to back up client photos to an external hard drive each week and also send some finished images to clients by email. The business is considering using compression software and encryption software as part of this process. Discuss how compression software and encryption software could each be used in this scenario, and evaluate whether the business needs both, one, or neither.

(Total for Question 15 is 6 marks)

Two scheduling algorithms an operating system can use to decide which process the processor works on next are round robin and first come first served (FCFS).

(a)Describe how each of these two scheduling algorithms decides which process to run.(2)

(b)A school computer runs many short background tasks (e.g. antivirus checks) alongside one long task the user is actively using (e.g. video editing). Justify which of the two scheduling algorithms would give the user a better experience, and why.(2)

(Total for Question 16 is 4 marks)

Strong encryption software is now built into most smartphones, meaning that even the manufacturer cannot read a user's messages or files without the user's key/passcode. Discuss the ethical issues raised by this level of encryption, considering both the benefits to individual users and the concerns raised by the police and security services.

(Total for Question 17 is 6 marks)

State two examples of utility software, other than encryption, defragmentation and compression software.

(Total for Question 18 is 2 marks)

Explain the purpose of user management within an operating system.

(Total for Question 19 is 3 marks)

Describe the purpose of a directory (folder) structure as part of an operating system's file management.

(Total for Question 20 is 2 marks)

A 20 MB video file is compressed using lossy compression to a compression ratio of 5:1. Calculate the size of the compressed file, in MB.

(Total for Question 21 is 3 marks)

A computer receives four interrupt signals at almost the same moment: a mouse click, a keyboard key press, a critical low battery warning, and a request from the printer to say it has run out of paper. State the order, from highest priority to lowest, in which the operating system should service these four interrupts, and justify your ordering.

(Total for Question 22 is 3 marks)

The pseudocode below simulates a simplified FIFO page replacement policy for a system with 3 physical memory frames. When a referenced page is not already held in one of the frames (a page fault), the oldest page is removed and the new page is added:

frames <- ["-", "-", "-"]
pageRefs <- [1, 2, 3, 1, 4, 1]
FOR i <- 1 TO 6
  page <- pageRefs[i]
  IF page NOT IN frames THEN
    FOR j <- 1 TO 2
      frames[j] <- frames[j + 1]
    NEXT j
    frames[3] <- page
  ENDIF
  OUTPUT i, frames[1], frames[2], frames[3]
NEXT i

(a)Complete the trace table for i = 1 to 5. Row 6 has been completed for you.

i	frames[1]	frames[2]	frames[3]
1	?	?	?
2	?	?	?
3	?	?	?
4	?	?	?
5	?	?	?
6	3	4	1

(5)

(Total for Question 23 is 5 marks)

Explain how the operating system uses both device drivers and interrupts together to manage input from a keyboard.

(Total for Question 24 is 5 marks)

Mark scheme · P1.4 System Software

Question 1

B1 any one valid function, e.g. memory management oe
B1 any second, different valid function, e.g. multitasking / peripheral management (device drivers) / user management / file management / providing the user interface oe
Answer: Any two from: memory management; multitasking; peripheral management (via device drivers); user management; file management; providing the user interface.

Question 2

B1 C cao
Answer: C

Question 3

B1 Row 1: Defragmentation software cao
B1 Row 2: Encryption software cao
B1 Row 3: Compression software cao
Answer: 1) Defragmentation software 2) Encryption software 3) Compression software

Question 4

B1 a device driver is translation software written specifically for that peripheral device oe
B1 it converts general commands/data from the operating system into the specific signals the printer's hardware understands (and vice versa) oe
B1 without the correct driver installed the operating system cannot send/receive data correctly to/from the printer oe
Answer: The driver acts as translation software written for that specific printer; it converts the general commands sent by the operating system into the specific signals the printer's hardware understands, and translates any data returned by the printer back for the operating system, allowing the OS to control the printer correctly without needing to know its internal workings.

Question 5

B1 an interrupt is a signal sent to the processor indicating that immediate attention is required oe
B1 the processor pauses the current process (after saving its state) to service the interrupt oe
B1 priority levels ensure more urgent interrupts (e.g. a power failure) are dealt with before less urgent ones (e.g. a printer running out of paper), rather than servicing them purely in the order they arrive oe
Answer: An interrupt is a signal sent to the processor to request immediate attention, causing it to pause the current process (after saving its state) and run an interrupt service routine. Interrupts are given priority levels so that more urgent interrupts, such as a power failure, are always serviced before less urgent ones, such as a printer running low on paper, regardless of the order in which they occur.

Question 6

B1 the operating system allocates a separate area of RAM to each open application/process oe
B1 it keeps track of which areas of memory are free or in use, preventing one application from overwriting another's data oe
B1 if RAM becomes full, the OS can move data between RAM and virtual memory on secondary storage, allowing more applications to appear to run at once oe
B1 a relevant example is given, e.g. a student streaming music while writing a document, each given its own memory space so both continue running without crashing or corrupting each other oe
Answer: The operating system allocates a separate, protected area of RAM to each application that is opened, for example a music streaming app and a word processor. It keeps track of which parts of memory are free or in use so that one application's data cannot overwrite another's. If RAM becomes full, the OS can move less-used data out to virtual memory on the hard disk/SSD, freeing RAM so more applications can keep running, appearing to run simultaneously.

Question 7

(a) B1 an area of secondary storage (e.g. the hard disk/SSD) that is used as if it were extra RAM oe
(a) B1 used by the operating system when RAM becomes full, to hold data/pages temporarily oe
(a) Answer: Virtual memory is a portion of secondary storage, such as the hard disk or SSD, that the operating system uses as if it were extra RAM, temporarily holding data when RAM is full.
(b) B1 RAM has become full, so the OS is repeatedly swapping (paging) data between RAM and virtual memory on the hard disk/SSD oe
(b) B1 this constant swapping is known as thrashing oe
(b) B1 because secondary storage is much slower to access than RAM, the constant swapping significantly increases the time taken to process tasks, slowing the whole system down oe
(b) Answer: RAM has filled up, so the operating system is constantly swapping (paging) data back and forth between RAM and virtual memory on the hard disk/SSD, a state known as thrashing. Because secondary storage is much slower to read from and write to than RAM, this constant swapping greatly increases access times and makes the whole system appear to slow down.

Question 8

B1 lossy compression permanently removes some data/detail from the file to reduce its size, so the file cannot be restored to its exact original oe
B1 example use of lossy compression, e.g. streaming music/video over the internet, where a small loss in quality is acceptable in exchange for a much smaller file oe
B1 lossless compression reduces file size without removing any data, so the file can be restored exactly to its original oe
B1 example use of lossless compression, e.g. compressing a text document or a database backup, where every bit of data must be recovered exactly oe
Answer: Lossy compression permanently discards some of the original data to achieve a much smaller file size, so the file cannot be restored exactly, for example streaming music or video, where a slight reduction in quality is an acceptable trade-off for faster streaming. Lossless compression reduces file size without discarding any data, so the original file can be perfectly restored, for example compressing a text document or database backup, where every byte must be recoverable exactly.

Question 9

(a) M1 correct identification of the three runs (5W, 3B, 4W)
(a) A1 correctly written as (W,5)(B,3)(W,4), or equivalent notation such as W5B3W4, cao
(a) Answer: (W,5)(B,3)(W,4) i.e. W5 B3 W4
(b) M1 compressed size = 3 pairs x 2 bytes = 6 bytes
(b) A1 ratio = 12:6 = 2:1 cao
(b) Answer: 2:1
(c) B1 RLE works well here because the image has long runs of the same repeated colour/value next to each other oe
(c) B1 a busy street photo has colours changing frequently between neighbouring pixels, so there are few or no long runs, meaning RLE would give little or no reduction in size (and may even increase it) oe
(c) Answer: RLE is effective here because the row contains long runs of identical pixel values next to each other. A photograph of a busy street has pixel colours that change frequently between neighbouring pixels, with few long runs of the same colour, so RLE would find very few repeated runs to compress, giving little or no reduction in file size.

Question 10

(a) B1 Round 1 row correct: 4, 1, 3 (all three values, cao)
(a) B1 Round 2 row correct: 2, 0, 1 (all three values, cao)
(a) B1 Round 3 row correct: 0, 0, 0 (all three values, cao)
(a) Answer: Round 1: 4, 1, 3 Round 2: 2, 0, 1 Round 3: 0, 0, 0
(b) B1 3 rounds cao
(b) B1 the loop condition checks whether tasks[0] > 0 OR tasks[1] > 0 OR tasks[2] > 0 oe
(b) B1 after round 3 all three values are 0, so every part of the OR condition is false, meaning the whole condition is false and the loop stops oe
(b) Answer: After 3 rounds. The loop condition checks tasks[0] > 0 OR tasks[1] > 0 OR tasks[2] > 0; once all three values reach 0 after round 3, every part of this condition is false, so the whole OR condition is false and the WHILE loop stops.

Question 11

B1 plain text is converted into cipher text using an algorithm and a (secret) key, so only someone with the key can decrypt it back to plain text oe
B1 the same key is used both to encrypt and to decrypt the data oe
B1 the key itself must somehow be shared with the recipient, and if this key is intercepted while being sent, an attacker can then decrypt the data oe
Answer: In symmetric encryption, plain text is converted into unreadable cipher text using an encryption algorithm and a secret key. The same key is then used to decrypt the cipher text back into plain text. The main problem is that this same key must be securely shared with the recipient; if the key is intercepted while being sent, an attacker who obtains it can decrypt the data.

Question 12

1 correctly inputs a string from the user
1 initialises a counter (to 1) and an empty result/output string before the loop
1 iterates through the string comparing each character to the next one, with correct loop bounds (e.g. 1 to length - 1)
1 correctly increments the counter when consecutive characters match
1 when characters differ, appends the character and its count to the result and resets the counter to 1
1 outputs the final result, ensuring the last run (after the loop ends) is correctly included
note award marks for any correct logic; ignore minor syntax errors that do not affect the logic

Answer:

INPUT text
count <- 1
result <- ""
FOR i <- 1 TO LEN(text) - 1
  IF MID(text, i, 1) = MID(text, i + 1, 1) THEN
    count <- count + 1
  ELSE
    result <- result + MID(text, i, 1) + STR(count)
    count <- 1
  ENDIF
NEXT i
result <- result + MID(text, LEN(text), 1) + STR(count)
OUTPUT result

Question 13

B1 on an HDD, files are often stored in fragments scattered across different physical locations, and the read/write head has to physically move to each location to read the whole file oe
B1 defragmentation reorganises the fragments so each file's data is stored together (contiguously), reducing the physical head movement needed and so reducing access time oe
B1 an SSD has no moving parts and uses flash memory, so it can access any memory location directly (near-instantly) regardless of where data is stored, so fragmentation does not slow it down oe
B1 SSD memory cells have a limited number of write cycles, so needlessly defragmenting (which involves extra write operations) can wear out the SSD faster and reduce its lifespan oe
Answer: On an HDD, fragmented files are scattered across different physical locations, so the read/write head must physically move between them, taking extra time. Defragmentation groups each file's fragments together, reducing head movement and so access time. An SSD has no moving parts and can access any location directly and near-instantly, so fragmentation does not slow it down; because flash memory cells have a limited number of write cycles, needlessly defragmenting an SSD causes extra writes that can shorten its lifespan.

Question 14

(a) B1 44 cao
(a) Answer: 44
(b) M1 correct place values used (128 64 32 16 8 4 2 1) and 44 = 32 + 8 + 4
(b) A1 00101100 shown as the final answer, cso (answer given in the question)
(b) Answer: 00101100

Question 15

Level 3 (5-6): A detailed, well-applied discussion of both compression and encryption, correctly applied to the scenario. A reasoned evaluation/judgement is given on whether both are needed, with justification.
Level 2 (3-4): A relevant explanation of how compression and/or encryption could be used in the scenario, with some application to the business's needs. Some linkage between points, mostly relevant.
Level 1 (1-2): Basic, isolated statements about compression and/or encryption with little or no application to the scenario. Limited or no use of technical terminology.
Level 0 (0): No creditable content.
Indicative content:
- compression could reduce the file size of images to make emailing faster/practical for clients, likely using lossy compression as a small quality loss is acceptable for previews
- lossless compression could be used for the backed-up master/original files so no image quality is lost, since these may be needed for future high-quality prints
- encryption could protect client photos on the external hard drive and during emailing, so client/personal data cannot be read if the drive is lost or stolen, or if the email is intercepted
- a reasoned judgement, e.g. that the business likely needs both compression (for practical file sizes and speed) and encryption (for confidentiality and legal reasons), because they solve different problems
- recognition of a trade-off, e.g. that encrypted files usually cannot be compressed as effectively, so compression should ideally be applied before encryption

Question 16

(a) B1 FCFS: processes are run strictly in the order they arrive in the queue, each running to completion before the next starts oe
(a) B1 round robin: each process is given a fixed time slice (quantum) of processor time in turn, moving to the back of the queue if not finished, cycling repeatedly oe
(a) Answer: FCFS runs processes strictly in the order they arrive in the queue, each one running to completion before the next begins. Round robin gives each process a fixed time slice of processor time in turn; if a process is not finished at the end of its slice it is moved to the back of the queue, and the cycle repeats.
(b) B1 round robin cao
(b) B1 valid justification, e.g. with FCFS the long video editing task could run first and block/delay all the short background tasks until it finishes, whereas round robin regularly gives each task, including the short ones, a slice of processor time so nothing is completely blocked, keeping the system responsive oe
(b) Answer: Round robin. With FCFS, if the long video editing task is running, all the short background tasks would have to wait until it completely finishes, which could take a long time. Round robin gives every process, including the short tasks, a regular time slice, so the system stays responsive and no single task can block the others for long.

Question 17

Level 3 (5-6): A well-balanced, developed discussion referencing both perspectives with clear links to the smartphone context. May include a reasoned conclusion/judgement weighing the two sides.
Level 2 (3-4): Some balanced discussion covering benefits to users and concerns for law enforcement, but points may be underdeveloped or not fully linked to the smartphone encryption scenario.
Level 1 (1-2): Basic, one-sided or generic statements about encryption being 'good' or 'bad', with little reference to both individual users and police/security services.
Level 0 (0): No creditable content.
Indicative content:
- benefit: protects users' personal data (photos, banking apps, messages) from criminals if the phone is lost or stolen, and from hackers
- benefit: protects privacy and freedom of expression, e.g. for journalists or activists communicating safely
- concern: police/security services may be legally unable to access a criminal suspect's phone data even with a warrant, hampering investigations into serious crimes
- concern: manufacturers cannot comply with law enforcement requests for data because they themselves cannot decrypt it (no back door)
- recognition of the tension between individual privacy rights and collective/public safety, potentially with a reasoned personal judgement or balanced conclusion

Question 18

B1 any one valid different example, e.g. anti-malware/anti-virus software / backup software / disk clean-up software / firewall software oe
B1 any second, different valid example from the same list oe
Answer: Any two from: anti-malware/anti-virus software; backup software; disk clean-up software; firewall software.

Question 19

B1 allows different user accounts to be set up on the same computer, each usually requiring a username and password to log in oe
B1 allows each user to have their own personal settings/files, kept separate/private from other users oe
B1 allows different levels of access/permissions to be set for different users, e.g. an administrator account can install software or change settings that a standard user account cannot oe
Answer: User management allows separate user accounts to be created on the same computer, typically requiring a username and password to log in. Each account can have its own personal files and settings, kept private from other users. It also allows different levels of access to be given to different accounts, for example an administrator account can install software or change system settings, which a standard user account is not permitted to do.

Question 20

B1 allows files to be organised/grouped in a hierarchical/logical structure of folders and sub-folders oe
B1 makes it easier for the user to find, access and manage files, rather than every file being stored in one place oe
Answer: A directory structure lets the operating system organise files into folders and sub-folders in a logical, hierarchical way, making it easier for users to find, access and manage their files rather than storing everything in one single location.

Question 21

M1 recognises that a ratio of 5:1 means compressed size = original size / 5
M1 20 / 5 shown as method
A1 4 MB cao
Answer: 4 MB

Question 22

B1 critical low battery warning placed first (highest priority) cao
B1 printer out of paper placed last (lowest priority), keyboard and mouse in between (either order accepted) cao
B1 valid justification linking urgency to consequence of delay, e.g. the battery warning risks losing power and unsaved data if not dealt with immediately, whereas nothing is lost by delaying the printer message, and user input delays are noticeable but far less critical than losing power oe
Answer: 1) critical low battery warning, 2) keyboard key press / mouse click (either order), 3) printer out of paper. The battery warning is serviced first because the computer risks losing power and unsaved data if it is not dealt with immediately. The printer's out-of-paper message is serviced last because no data is lost or work interrupted while it waits; keyboard and mouse input are given quicker attention than the printer notice because delays to user input are noticeable, but they are far less urgent than the risk of total power loss.

Question 23

(a) B1 i=1 row correct: -, -, 1 (page fault, all three values cao)
(a) B1 i=2 row correct: -, 1, 2 (page fault, all three values cao)
(a) B1 i=3 row correct: 1, 2, 3 (page fault, all three values cao)
(a) B1 i=4 row correct: 1, 2, 3 (page 1 already in frames, no shift, all three values cao)
(a) B1 i=5 row correct: 2, 3, 4 (page fault, all three values cao)
(a) Answer: i=1: -, -, 1 i=2: -, 1, 2 i=3: 1, 2, 3 i=4: 1, 2, 3 i=5: 2, 3, 4 (i=6: 3, 4, 1)

Question 24

B1 when a key is pressed, the keyboard sends an interrupt signal to the processor oe
B1 the processor pauses/suspends whatever process it is currently running and saves its current state, so it can return to it later oe
B1 the processor then runs the interrupt service routine associated with the keyboard oe
B1 the keyboard's device driver is used within this routine to translate the raw signal from the keyboard hardware into a form the operating system/application can use, e.g. the correct character code oe
B1 once the keyboard input has been dealt with, the processor resumes the paused process from where it left off, using the saved state oe
Answer: When a key is pressed, the keyboard sends an interrupt signal to the processor. The processor pauses whatever it is currently doing, saving its current state, and runs the interrupt service routine for the keyboard. Within this routine, the keyboard's device driver translates the raw hardware signal into a form the operating system and application can use, such as the correct character code. Once this has been dealt with, the processor resumes the previously paused process from exactly where it left off, using the state that was saved.

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