History

• The first computers did not have operating systems. By the early 1960s, commercial computer vendors were supplying quite extensive tools for streamlining the development, scheduling, and execution of jobs on batch processing systems. Examples were produced by UNIVAC and Control Data Corporation, amongst others.

• MS-DOS provided many operating system like features, such as disk access. However many DOS programs bypassed it entirely and ran directly on hardware.

• The operating systems originally deployed on mainframes, and, much later, the original microcomputer operating systems, only supported one program at a time, requiring only a very basic scheduler. Each program was in complete control of the machine while it was running. Multitasking (timesharing) first came to mainframes in the 1960s.

• In 1969-70, UNIX first appeared on the PDP-7 and later the PDP-11. It soon became capable of providing cross-platform time sharing using preemptive multitasking, advanced memory management, memory protection, and a host of other advanced features. UNIX soon gained popularity as an operating system for mainframes and minicomputers alike.

• IBM microcomputers, including the IBM PC and the IBM PC XT could run Microsoft Xenix, a UNIX-like operating system from the early 1980s. Xenix was heavily marketed by Microsoft as a multi-user alternative to its single user MS-DOS operating system. The CPUs of these personal computer, could not facilitate kernel memory protection or provide dual mode operation, so Microsoft Xenix relied on cooperative multitasking and had no protected memory.

• The 80286-based IBM PC AT was the first computer technically capable of using dual mode operation, and providing memory protection.

• Classic Mac OS, and Microsoft Windows 1.0-Me supported only cooperative multitasking, and were very limited in their abilities to take advantage of protected memory. Application programs running on these operating systems must yield CPU time to the scheduler when they are not using it, either by default, or by calling a function.

• Windows NT's underlying operating system kernel which was a designed by essentially the same team as Digital Equipment Corporation's VMS, a UNIX-like operating system which provided protected mode operation for all user programs, kernel memory protection, preemptive multi-tasking, virtual file system support, and a host of other features.

• Classic AmigaOS and Windows 1.0-Me did not properly track resources allocated by processes at runtime. If a process had to be terminated, the resources might not be freed up for new programs until the machine was restarted.