Sample questions from Systems Software

Answer each one closed book first, then open the answer.

1. Operating systems: need, function and purpose

   How does an operating system provide a 'platform' for application software?

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   It exposes a consistent set of services and interfaces (often called an application programming interface) that applications call instead of controlling hardware. A word processor asks the operating system to save a file; it does not need to know whether the storage is a hard disk drive, a solid state drive or a network share.

2. Memory management: paging, segmentation and virtual memory

   What is virtual memory?

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   A technique in which a section of secondary storage is used as though it were RAM. Pages that are not currently needed are written out to this area, freeing real RAM for the pages that are needed, so programs larger than physical memory can run and more programs can be open at once than would otherwise fit.

3. Memory management: paging, segmentation and virtual memory

   A machine has 8 KiB of RAM available to processes and a page size of 1 KiB. A process needs 5.2 KiB. How many frames does it occupy and how much space is wasted?

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   5.2 KiB requires 6 pages, because a partly used page still needs a whole frame, so it occupies 6 frames of 1 KiB. The wasted space is 6 - 5.2 = 0.8 KiB, sitting unused inside the last page. This waste inside an allocated page is internal fragmentation.

4. Interrupts and the interrupt service routine

   What is an interrupt service routine?

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   A block of code, held in memory as part of the operating system or a device driver, that carries out the action needed to deal with a particular type of interrupt. Each type of interrupt has its own routine, and the addresses of these routines are held in an interrupt vector table so the correct one can be found quickly.

5. Interrupts and the interrupt service routine

   What is the interrupt vector table?

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   A table in memory holding the start address of the interrupt service routine for each type of interrupt. When an interrupt is accepted, the processor uses the interrupt's identifying number as an index into this table to obtain the address to load into the program counter, so the correct handler is reached without searching.

6. Scheduling algorithms: first come first served, round robin and shortest job first

   How does round robin scheduling work?

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   Each process in the ready queue is given the processor for a fixed period called a time slice or quantum. If it has not finished when the quantum expires, a timer interrupt pre-empts it and it is placed at the back of the queue, and the next process runs. The queue is cycled through repeatedly until all processes have finished.

7. Scheduling algorithms: shortest remaining time, multi-level feedback queues, traces and choosing an algorithm

   Two processes are ready at time 0: X needs 10 ms and Y needs 2 ms. Calculate the average waiting time if X runs first, and if Y runs first.

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   If X runs first, X waits 0 and Y waits 10, so the average waiting time is 5 ms. If Y runs first, Y waits 0 and X waits 2, so the average waiting time is 1 ms. Running the shorter job first has cut the average waiting time to a fifth, which is why shortest job first minimises average waiting time.

8. Types of operating system

   Give one drawback of an embedded operating system.

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   It is inflexible: because it is written for one device and one task, it cannot easily be extended, and adding a feature usually means replacing the firmware or the whole device. Many embedded systems are also rarely patched, which leaves known security weaknesses in place for the life of the product.

The 7 subtopics

One subtopic is one session. Work down the list.

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

1. Step 1 · Closed book

   Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.

2. Step 2 · Open book

   Go back to the top. Read each printed answer and write it out in full, including the ones you had right.

3. Step 3 · Closed book again

   Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.

Nearby topics

All 16 topics · Guide overview