Operating System

Assignment #2 

30 Points

2.1. Suppose we have a multi-programmed computer in which each job has identical characteristics. In one computation period, T, for a job, half the time is spent in I/O, and the other half in processor activity. Each job runs for a total of N periods. Assume a simple round-robin scheduling is used, and I/O operations can overlap with processor operation. Define the following quantities:

§ Turnaround time = actual time = actual time to complete a job

§ Throughput=average Throughput = average number of jobs completed per time period T

§ Processor utilization=percentage utilization= percentage of time that the processor is active (not waiting)


Compute these quantities for one, two, and four simultaneous jobs, assuming that the period T is distributed in each of the following ways:

a. Processor first half, I/O second half

b. I/O first and third quarters, processor second and fourth quarter

2. Give the features of Batch Operating System and list the advantages and the disadvantages.

A batch operating system normally don’t interact with the computer directly. In between there is an operator which take similar jobs and groups them into batches.

3. Instructions related to accessing I/O devices are typically privileged instructions, that is, they can be executed in kernel mode but not in user mode. Give a reason why these instructions are privileged.

4. One reason GUIs were initially slow to be adopted was the cost of the hardware need-ed to support them. How much video RAM is needed to support a 25-line × 80-row character monochrome text screen? How much for a 1200 × 900-pixel 24-bit color bit-map? What was the cost of this RAM at 1980 prices ($5/KB)? How much is it now?

2.5. In IBM’s mainframe OS, OS/390, one of the major modules in the kernel is the System Resource Manager. This module is responsible for the allocation of resources among address spaces (processes). The SRM gives OS/390 a degree of sophistication unique among operating systems. No other mainframe OS, and certainly no other type of OS, can match the functions performed by SRM. The concept of resource includes processor, real memory, and I/O channels. SRM accumulates statistics pertaining to utilization of processor, channel, and various key data structures. Its purpose is to provide optimum performance based on performance monitoring and analysis. The installation sets forth various performance objectives, and these serve as guidance to the SRM, which dynamically modifies installation and job performance characteristics based on system utilization. In turn, the SRM provides reports that enable the trained operator to refine the configuration and parameter settings to improve user service. This problem concerns one example of SRM activity. Real memory is divided into equal-sized blocks called frames, of which there may be many thousands. Each frame can hold a block of virtual memory referred to as a page. SRM receives control approximately 20 times per second, and inspects each and every page frame. If the page has not been referenced or changed, a counter is incremented by 1. Over time, SRM averages these numbers to determine the average number of seconds that a page frame in the system goes untouched. What might be the purpose of this, and what action might SRM take?

2.6. A multiprocessor with eight processors has 20 attached tape drives. There is a large number of jobs submitted to the system that each require a maximum of four tape drives to complete execution. Assume each job starts running with only three tape drives for a long period before requiring the fourth tape drive for a short period toward the end of its operation. Also assume an endless supply of such jobs.

a. Assume the scheduler in the OS will not start a job unless there are four tape drives available. When a job is started, four drives are assigned immediately and are not released until the job finishes. What is the maximum number of jobs that can be in progress at once? What are the maximum and minimum number of tape drives that may be left idle as a result of this policy?

b. Suggest an alternative policy to improve tape drive utilization and at the same time avoid system deadlock. What is the maximum number of jobs that can be in progress at once? What are the bounds on the number of idling tape drives?

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