|Idioms and phrases|
An operating system is a program that acts as an intermediary between the user and the computer hardware. The purpose of an OS is to provide a convenient environment in which user can execute programs in a convenient and efficient manner. It is a resource allocator responsible for allocating system resources and a control program which controls the operation of the computer hardware.
It is a useful, memory-saving technique for multiprogrammed timesharing
systems. A Reentrant Procedure is one in which multiple users can share a single copy of
a program during the same period.
Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.
A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.
In batched operating system the users gives their jobs to the operator who sorts the programs according to their requirements and executes them. This is time consuming but makes the CPU busy all the time.
Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Beladys Anomaly. This is true for certain page reference patterns.
The machine purpose workstation individual usability &resources utilization mainframe optimize utilization of hardware PC support complex games, business application Hand held PCs Easy interface & min. power consumption.
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