1.5 In a multiprogramming and time-sharing environment, several users share the system simultaneously.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 humans can inevitably be broken by a human, and the more complex the scheme, the more difficult it is to feel confident of its correct implementation.
1.6 Define the essential properties of the following types of operating systems: a. Batch
c. Time sharing
d. Real time
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. This system is composed of many short transactions where the results of the next transactionmay be unpredictable. Response time needs to be short (seconds) since the user submits and waits for the result. c. Time sharing. This systems uses CPU scheduling and multiprogramming to provide economical interactive use of a system. The CPU switches rapidly from one user to another. Instead 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, this system reads information from sensors and must respond within a fixed amount of time to ensure correct performance. e....
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