In a multiprogramming and time-sharing environment, several users share the system si-multaneously. This situation can result in various security problems.
a. What are two such problems?
b. Can we ensure the same degree of security in a time-shared machine as we have in a dedicated machine? Explain your answer.
Answer:
a. Stealing or copying one's programs or data; using system resources (CPU, memory, disk space, peripherals) without proper accounting.
b. Probably not, since any protection scheme devised by man can inevitably be broken by him, and the more complex the scheme, the more difficult it is to feel confident of its correct implementation.
What is the main advantage of multiprogramming?
Answer: Multiprogramming makes efficient use of the CPU by overlapping the demands for the CPU and its I/O devices from various users. It attempts to increase CPU utilization by always having something for the CPU to execute.
1.6 What are the main differences between operating systems for mainframe computers and personal computers?
Answer: Personal computer operating systems are not concerned with fair use, or maxi-mal use, of computer facilities. Instead, they try to optimize the usefulness of the computer for an individual user, usually at the expense of efficiency. Consider how many CPU cy-cles are used by graphical user interfaces (GUIs). Mainframe operating systems need more complex scheduling and I/O algorithms to keep the various system components busy.
Define the essential properties of the following types of operating systems:
a. Batch
b. Interactive
c. Time sharing
d. Real time
e. Distributed
Answer:
a. Batch. Jobs with similar needs are batched together and run through the computer as a group by an operator or automatic job sequencer. Performance is increased by attempting to keep CPU and I/O devices busy at all times through buffering, off-line operation, spooling, and multiprogramming. Batch is good for executing large jobs that need little interaction; it can be submitted and picked up later.
b. Interactive. Composed of many short transactions where the results of the next trans-action may be unpredictable. Response time needs to be short (seconds) since the user submits and waits for the result.
c. Time sharing. Uses CPU scheduling and multiprogramming to provide economical interactive use of a system. The CPU switches rapidly from one user to another. In-stead of having a job defined by spooled card images, each program reads its next control card from the terminal, and output is normally printed immediately to the screen.
d. Real time. Often used in a dedicated application. The system reads information from sensors and must respond within a fixed amount of time to ensure correct perfor-mance.
e. Distributed. Distributes computation among several physical processors. The pro-cessors do not share memory or a clock. Instead, each processor has its own local memory. They communicate with each other through various communication lines, such as a high-speed bus or telephone line.
We have stressed the need for an operating system to make efficient use of the computing hardware. When is it appropriate for the operating system to forsake this principle and to " waste" resources? Why is such a system not really wasteful?
Answer: Single-user systems should maximize use of the system for the user. A GUI might " waste" CPU cycles but it optimizes the user's interaction with the system.
Under what circumstances would a user be better off using a time-sharing system, rather than a personal computer or single-user workstation?
Answer: When there are few other users, the task is large, and the hardware is fast, time-sharing makes sense. The full power of the system can be brought to bear on the user's problem. The problem can be solved faster than on a personal computer. Another case is when there are lots of other users needing resources at the same time.
A personal computer is best when the job is small enough to be executed reasonably on it, and when performance is sufficient to execute the program to the user's satisfaction.
Describe the differences between symmetric and asymmetric multiprocessing. What are three advantages and one disadvantage of multiprocessor systems?
Answer: Symmetric multiprocessing treats all processors as equals, and I/O can be pro-cessed on any CPU. Asymmetric multiprocessing has one master CPU and the remain-der CPUs are slaves. The master distributes tasks among the slaves, and I/O is usually done by the master only. Multiprocessors can save money, by not duplicating power sup-plies, housings, and peripherals. They can execute programs more quickly, and can have increased reliability. They are also more complex in both hardware and software than uniprocessor systems.
Why are distributed systems desirable?
Answer: Distributed systems can provide resource sharing, computation speedup, in-creased reliability, and the ability to communicate with remote sites.
