Unit 2 - Systems Software

Question 1

What are the tasks of the OS? (5)

Answer 1

• Managing processor: chooses which process to execute and handles interrupts
• Managing memory: uses paging and segmentation
• Manages peripherals: uses device drivers to communicate with hardware
• Manages security: accepts/denies requests to access resources connected to the computer
• Provides networking: communication through protocols to other devices

Question 2

What are the differences between paging and segmentation? (3)

Answer 2

• Paging divides memory into fixed-size chunks called pages; segmentation divides memory into varied-length chunks called segments.
• Pages are made to fit sections of memory; segments are complete sections of programs
• Pages have physical divisions; segments have logical divisions

Question 3

Explain how virtual RAM works.

Answer 3

• When the main memory is full
• Programs that are not currently in use
• Are sent to a section of secondary storage through paging
• To free up space in the main memory for other programs

Question 4

What is disk thrashing?

Answer 4

• When the computer slows down from pages being swapped too frequently between secondary storage and main memory.

Question 5

What is an interrupt and what are the types of interrupt?

Answer 5

• Interrupts are a signal sent by a device via the control bus to the CPU to indicate that it requires attention
• Hardware interrupt: power/reset button is pressed or memory is corrupted
• Software interrupt: illegal instruction encountered, arithmetic overflow, new log-on request
• Input/Output: buffer nearly empty, signal the completion of data to/from a device

Question 6

What is the memory stack?

Answer 6

• A stack located in memory
• Which holds the contents of the register when an interrupt is detected
• Contents are reloaded back into the registers in a FILO order
• If a higher-priority interrupt is detected while another interrupt is being processed, the contents of the registers for the lower-priority interrupt is also pushed to the memory stack

Question 7

Explain the different types of scheduling.

Answer 7

• Round Robin: each job is allocated an equal time slice by using a FIFO order during which it can use the CPU’s resources
• First Come First Serve: jobs are executed in the order they arrive
• Shortest Job First - the processor picks the job that the scheduler knows will take the least amount of time and processes it until it is completed
• Shortest Time Remaining: switches to and runs the job the scheduler knows will take the least time. If a shorter process arrives, it will switch to it.

Question 8

What are the differences between open-source and proprietary software?

Answer 8

• Open source is usually free; proprietary is usually not free
• Open source is less secure; proprietary is more secure
• Open source allows users to see and modify source code; proprietary hides source code from users
• Users own the open source code once they obtain it; users only own a licence to the source code when purchasing propriety software

Question 9

What are the differences between interpreters and compilers?

Answer 9

• Interpreters execute lines sequentially until they hit an error; compilers cannot execute if there is an error in the code
• Compilers translate high-level code to an executable file written in object code
• Once compiled, high-level code does not need to be recompiled to be run; high-level code must be interpreted each time it is run

Question 10

Explain the 4 stages of compilation.

Answer 10

Lexical Analysis:

1) Whitespace is removed
2) Code is tokenised
3) Symbol table is created

Syntax Analysis:

4) Syntax is checked against language rules
5) Abstract syntax tree created
6) Errors are generated

Code Generation:

7) Syntax tree is converted to object code

Code Optimisation:
8) Code is optimised, redundant code is removed