Needed by operating system to implement its functions (for instance to implement resume and other system calls).
In Xinu it consists of disabling interrupts and not making any call that may result in rescheduling.
Major drawbacks of this technique as a general tool for mutual exclusion:
Allows only one activity on a single computer. Thus it stops all processes when one of them executes the critical section.
Disables clock and other device interrupts. Thus lengthy computations in a critical section are not desirable.
Technique fails on a multiprocessor system.