LET YOUR MU-C TACKLE SEVERAL PROGRAMS

An operating system 9OS0 is a set of prewritten programs that reside in memory along with the user-written application programs. Typically, an operating system is not highly dependent upon the details of the specific application. Nevertheless, there have evolved two distinctly different types of system: disk operating systems 9DOS0, and multitasking operating systems 9MTOS0. The DOS is designed to support a user who is preparing, debugging, testing, or running a program on a general-purpose computer. Usually, the DOS supports several peripherals (such as one or two disks, a printer, and one or more consoles), and a moderately generous memory. A DOS by itself is usually not sufficient for applications which have significant random, asynchronous inputs, or which must carry out several related subactivities essentially simultaneously. Applications of this general nature require the additional program-level services of a multi-tasking operating system. There is one crucial difference between the tasks of an MTOS-supported program and the sections of a DOS-supported program. Within the DOS-supported program there is a single execution point (the program counter) which moves through the program from beginning to end. Within the MTOS-supported program, each task has a separate and independent program counter; several tasks can be executing simultaneously. Because the computer has only one hardware CPU and program counter, only one task can actually be executing at any given instant. MTOS gives the cPU to the highest priority task that is ready to be executed, and gets it running. However, if the task reaches an inherent pause (such as waiting for disk access), the next-highest-priority task gets a chance to continue as long as it can, during the pause.