Summary

An operating system (OS) is a software program. The operating system is an example of system

software. Operating systems manage computer system hardware and software. Some examples of

operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS,

VMS, MVS, and VM.

A virtual machine is an imaginary machine that is created by Kernel. Kernel presents each virtual

machine with an image resembling the hardware of the real machine.

There are two primary benefits of using virtual machine: First the virtual machine provides more security.

Second the virtual machine allows system development to be done without disrupting normal system

operation.

Concurrency is a property of systems in which several computations are executing simultaneously.

Deadlock occurs when two processes need two non-shareable resources to proceed but one process

has acquired one resource and the other has acquired the other resource.

Virtual memory is a technique that allows the execution of processes that may not be completely in

memory. Virtual memory does not physically exist on a memory chip but it is implemented by the

operating system in order to permit multitasking. Virtual memory can be implemented by a technique

called paging.

Summary

A process is a program in execution. A process is an instant of a program in execution.

There are five possible states of a process.

For each process, its process control block PCB is used to track the process's execution status.

Process scheduling means changing the active PCB pointed to by the CPU.

Interrupts are special signals sent by hardware or software to the CPU.

Spool refers to the process of placing data in a temporary working area for another program to process.

Time-sharing is the sharing of a computing resource among many users by means of multiprogramming

and multitasking.

Time slicing is the most common technique to implement time sharing in operating systems.