OS- Operating System.

OPERATING SYSTEM:

An operating system (OS) is software that manages computer hardware and software resources and provides common services for computer programs. The operating system is an essential component of the system software in a computer system. Application programs usually require an operating system to function.

For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and will frequently make a system call to an OS function or be interrupted by it. Operating systems can be found on almost any device that contains a computer—from cellular phones and video game consoles to supercomputers and web servers.
UNIX  OS:

• Developed in the early 1970s at Bell Laboratories by Ken Thompson and Dennis Ritchie.
• Written in C high level language hence highly portable.
• Multi-user, time sharing OS.
• Used on a variety of computers ranging from notebook computers to super computers.
• Especially prevalent on RISC workstations such as those from Sun Microsystems, IBM and Silicon graphics
• Structured in three layers- kernel,shell and utilities.

MS-DOS:

• Stands for Microsoft Disk Operating System.
• Single user OS for IBM and IBM compatible personal cmputers.
• Structured in three layers BIOS (Basic Input Output System), kerneel, and shell.
• Very popular in 1980's, now not in much use and development of Microsoft Windows OS in 1990s.

LINUX:

• Open-source OS enhanced and backed by thousand of programmer world wide.
• Multi-Tasking, multiprocessing OS, originally designed to be used in PCs.
• Several Linux distributions available (Red Hat,SuSE). Difference in distribution is mostly set of tools, number and quality of applications, documentation, support, and service.