星期四, 10月 08, 2009

CS3161 (A) OPERATING SYSTEM PRINCIPLES (DR. LIU WENYIN) (09CS3161_LW) - Answers to Tutorial 3 Questions

Q1 What is PCB? What information are usually stored in PCB?

PCB Stands for Process Control Block.

Process State
Program Counter
CPU Registers
CPU Scheduling information
Memory management information
I/O Status
Accounting information



Q2 Explain the concept of a context switch.

Whenever the CPU starts executing a new process, the old process's state must be preserved. The context of a process is represented by its process control block. Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process. This task is known as a context switch. When a context switch occurs, the kernel saves the context of the old process in its PCB and loads the context of the new process scheduled to run.


Q3 Explain the main differences between a short-term and long-term scheduler.

The primary distinction between the two schedulers lies in the frequency of execution. The short-term scheduler is designed to frequently select a new process for the CPU, at least once every 100 milliseconds. Because of the short time between executions, the short-term scheduler must be fast. The long-term scheduler executes much less frequently; minutes may separate the creation of one new process and the next. The long-term scheduler controls the degree of multiprogramming. Because of the longer interval between executions, the long-term scheduler can afford to take more time to decide which process should be selected for execution.



Q4 We can describe much of processor management in terms of process state transition diagrams, such as:

Run 2 1 3 4 Wait Ready (Blocked)

(i) Give one example of "event" that causes each of the mark transitions ?
(ii) When we view all the processes in the system, we can see that a state transition by one process could cause another process to make a state transition also. Under what circumstances could transition 3 by one process immediately cause transition 1 by another process ? List all similar situations.


Q4 Process State Transitions

(i) Transition 1 - Last process completed or blocked, another process on the ready queue will be allocated the CPU for execution (Dispatch).
Transition 2 - Preemptive scheduling system
Transition 3 - Process requests an I/O operation
Transition 4 - I/O device completion, process joins the ready queue.

(ii) Process Transitions :
Process request I/O operation - When a process make an I/O service request (Transition 3), when the I/O device is not available at that time, it will go through a transition from run state to wait (blocked) state. At the same time another process on the ready queue will be allocated the CPU for execution (Transition 1) during that time.

Process exceeded the CPU allowance, completed execution or abortion in error condition (Transition 2), another process dispatch to use the CPU (Transition 1).

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