CITS2230 Operating Systems Mid-semester test 2006

Transcription

1 School of Computer Science & Software Engineering The University of Western Australia CITS2230 Operating Systems Mid-semester test 2006 Time allowed: 45 minutes, no reading period You may not communicate with anyone except the invigilators during this test You may not refer to any books or notes during this test Write your answers in the spaces provided on the question sheet This test has 5 pages and 10 questions Attempt every question This test is worth 20% of CITS2230 Full name: Student number: DO NOT WRITE IN THIS SPACE CITS2230 Mid-semester test of 5 September 2006

2 Q1. 2 marks List three distinct reasons why a process might leave the Ready state in an operating system using pre-emption, and which state it goes to in each case. the process is scheduled to run: to Running. the process is terminated by the OS or by its parent: to Exit. the process is suspended by the OS: to Ready-suspend. Q2. 2 marks What is the main goal of incorporating process suspension into an operating system? How does process suspension contribute to achieving this goal? The main goal is to keep the processor as busy as possible, by increasing the number of processes that might be available for execution. It does this by moving some blocked processes out to disk, freeing up main memory for other processes that aren t (currently) blocked. Q3. 2 marks With the LINUX fork() command, what is the aim of the copy-on-write protocol? How is this protocol implemented? The goal is to minimise the copying required by the semantics of the fork() command, especially where a fork() is quickly followed by a execve(). It is implemented by allowing both parent and child processes to refer to the same copy of each of the parent s pages until/unless one of them writes to that page. CITS2230 Mid-semester test of 5 September 2006

3 Q4. 2 marks List four requirements of a memory management scheme in an operating system. relocation: a process should be able to execute anywhere in memory. protection: a process s image should not be accessible by any other process. sharing: processes should be able to share parts of their image. logical organisation: the resolution of references from one module to another. physical org: the allocation of concrete resources to hold a process s image. Q5. 2 marks What is the main difference between paging of memory and segmentation of memory? List two ways in which this difference affects the address-translation hardware required with each scheme. All pages are the same size, whereas segments can vary in size. with segments, a bounds register is required, to check for illegal accesses. pages always start on an address that is a power of two, so address offsetting is a simple bit-concatenation; with segments, address offsetting is a true addition, and is slower. Q6. 2 marks Explain briefly each of the terms long-term scheduling, medium-term scheduling, and short-term scheduling. Long-term scheduling decides when a new process will be admitted to the system. Medium-term scheduling decides when a process will be swapped out of/into main memory. Short-term scheduling decides which ready process will be chosen to execute in the next time-slice. CITS2230 Mid-semester test of 5 September 2006

4 Q7. 2 marks Describe briefly two policies that a short-term scheduler might implement in order to promote I/O-bound processes over compute-bound processes. When a process doesn t use all of its time-slice (probably an I/O-bound process), it could put the process into an auxiliary queue and re-schedule the process for the remainder of its time-slice as soon as it is unblocked. When a process uses all of its time-slice (probably a compute-bound process), it could downgrade the priority of the process: thus (in the longer term) I/O-bound processes would typically have higher priority than compute-bound processes. Q8. 2 marks Explain briefly (i) the main advantage of interrupt-driven I/O over programmed I/O, and (ii) the main advantage of I/O using Direct Memory Access over interrupt-driven I/O. With interrupt-driven I/O, the processor can run other processes while the I/O is being performed; with programmed I/O, the processor busy-waits on the I/O operation. With I/O using DMA, the processor is interrupted only once, when the I/O operation is complete; with interrupt-driven I/O, the processor is interrupted each time a byte needs to be moved into memory. CITS2230 Mid-semester test of 5 September 2006

5 Q9. 2 marks Describe briefly the main difference between static linking and dynamic linking in compilation. Describe briefly two advantages of dynamic linking over static linking. With static linking, the compiler builds a self-contained executable at compile-time. With dynamic linking, it builds an executable that includes references to the original object files. smaller executables. modules can be shared easily. modules can be updated more easily, even for a running program. Q10. 2 marks Explain briefly the difference between multi-programming and multi-processing. Explain briefly how deadlock can arise in concurrent processing. Multi-programming has multiple processes on one processor. Multi-processing has multiple processes on multiple processors. Deadlock arises when two processes each hold a resource that the other needs to continue execution. CITS2230 Mid-semester test of 5 September 2006