Answer 8—A single CPU can switch contexts (threads or processes) via only two mechanisms: cooperatively via a system call or preemptively via an interrupt. Obviously, one would never make a context-switching system call in the middle of an atomic sequence, so the only other requirement is that interrupts be disabled for the duration of the sequence to prevent a preemptive context switch. This is known as a “critical section.” Care must be taken to keep critical sections as short as absolutely possible, because they affect the latency and jitter of all interrupts in the system.

Contributor: David Tweed