At the heart of the operating system is the multitasking Exec. It handles scheduling, interrupts, and the usual things one would expect. Exec is much smaller than the UNIX kernel, since many related features such as device drivers are relegated to another part of the system.
Unlike task switchers and ``cooperative multitasking'' systems found on other popular PC's, the Exec is a full-fledged preemptive multitasking system.