1.2.1 Systems Software

Question 1

What is the purpose of an operating system?

Answer 1

• Provide an interface between the user and computer’s hardware.
• Handles memory management, ISRs, processor scheduling and I/O.

Question 2

How do paging and segmentation function, and why is memory management needed?

Answer 2

• Paging splits memory physically into identical size pages, ready to be moved to and from storage.
• Segmentation splits memory logically, into different size segments based on program flow.
• Management allows programs larger than memory to run, protects programs from overwriting each other, and maps logical to physical memory addresses.

Question 3

What is the purpose of virtual memory?

Answer 3

• A section of secondary storage where less used programs are sent to when memory becomes full.
• Transfer between storage and memory is very slow.

Question 4

What are the steps of an interrupt service routine (ISR)?

Answer 4

• At the end of an FDE cycle, check interrupt register.
• If there is one and it is of a higher priority than the current job, dump registers into a memory stack.
• A flag is set and an ISR is loaded into memory.
• Once the interrupt is serviced, registers emptied from stack LIFO.
• Continue previous job (repeat cycle).

Question 5

What are the types of scheduling algorithm?

Answer 5

• Pre-emptive algorithms actively start and stop jobs (MLFQ, Shortest remaining time, Round Robin).
• Non-pre-emptive algorithms leave jobs to finish once they have begun (First come first served, shortest job first).

Question 6

How do the round robin and multilevel feedback queue scheduling algorithms work?

Answer 6

• Round robin gives each job an equal time slice and cycles through each until complete. All jobs are tended to, but long jobs will take some time.
• Multilevel feedback queues have several queues sorted by priority. Far better performance, but much harder to implement.

Question 7

How do the first come first serve, shortest job first, and least remaining time scheduling algorithms work?

Answer 7

• First come first serve tends to jobs in chronological order. It is very simple to implement but very inefficient to the point of starvation.
• Shortest job first handles jobs based on their overall size, reducing wait times, but requires calculations of job sizes and cannot prioritise.
• Least remaining time handles jobs based on how long is left. Increases throughput but has no regard for priority and can cause starvation.

Question 8

What are the types of operating system?

Answer 8

• Distributed systems run across several computers, splitting processing load.
• Embedded systems are made for small, specific tasks. Not powerful but efficient to run.
• Multitasking systems use time slices to give the illusion of running several programs at a time.
• Multi-user systems have several users with their own data and settings on a single device.
• Real-Time systems have a guaranteed response time for use in time-critical scenarios.

Question 9

What is the purpose of the BIOS?

Answer 9

• Performs Power-On-Self-Tests (POSTs) to ensure all hardware functions correctly (CPU clock, memory, processor).
• Tests any external memory devices to ensure function.
• Uses the bootstrap to load the OS into memory from storage.

Question 10

What is the purpose of device drivers?

Answer 10

• Programs provided by the OS that allow hardware to interact with it through requests, so the processor can handle the input.
• Specific to the computer’s architecture and OS.

Question 11

What is the purpose of a virtual machine?

Answer 11

• A software implementation of a computer system within another.
• Uses intermediate code (halfway between machine and object) that is not hardware-specific, and translators, to emulate other devices and their programs.
• Can be used as security from malware (cannot escape VM) or for emulating incompatible software.