What is an Operating system

    THE   OPERATING SYSTEM?

CONCEPTS OF OPERATING SYSTEMS

* System calls:

The interface between the operating system and the user programs is defined by the set of "extended instructions" that the operating system provides. These extended instructions are known as system calls. System calls vary from one operating system to another (although the fundamental concepts tend to be analogous).



Calls to the system are usually classified into two general categories: those that relate to processes and those that do to the file system.

* By Processes:

A process is basically a running program. It consists of the executable program and the stack or stack of the program, its program counter, stack pointer and other registers, as well as the other information that is needed to run the program. In itself the process in the concept of operating systems is like the timeshare system. That is, that periodically, the operating system decides to suspend the execution of one process and start the execution of another, for example, because the first one has already taken more of its part of the CPU time, in areas of the second. .



When a process is temporarily suspended, it must be restarted exactly in the same state it was in when it was stopped. This means that all information regarding the process must be explicitly saved somewhere during the suspension. In many operating systems, all the information relating to each process, different from the content of its address space, is stored in an operating system table called process table, which is an array (linked list) of structures, one for each process in existence.




Therefore, a process (suspended) consists of its address space, usually called the core image (in honor of the magnetic core image memories that were used in ancient times) and its record of the process table, which contains his records among other things.



* By File System:

An important function of the S.O. it consists of hiding the peculiarities of the disks and other I / O devices and presenting the programmer with an abstract, clean and pleasant model of files independent of the device. System calls are clearly needed to create files, delete them, read them and write them. Before a file can be read, it must be opened and after it has been read it must be closed, so that calls are given to do these things.




Before a file can be read or written, it must be opened, at which time the permissions are verified. If access is allowed, the system produces a small integer called a file descriptor to be used in subsequent operations. If access is prohibited, an error code is produced.