Monotasking systems, also referred to as single-tasking systems, are operating systems in which only one thread of execution is run at a given time. When an application is executed, it takes control of the whole computer, save for a 'resident' part of the OS which handles system calls and reloads the rest of the system when the application exits. Generally speaking, such systems have little or no protection against malicious software.
The canonical example of a monotasking operating system is MS-DOS.
While they are easier to design and write, monotasking operating systems are extremely limited. As a result, nearly all new OS designs are forMultitasking Systems.