An
instruction for
x86 processors. HLT halts the processor until the
RESET line is activated or an
interrupt or
NMI is received. It takes two
cycles on an
8088, two on a
186, two on a
286, five on a
386, four on a
486, and four on a
586.
When the processor isn't doing a lot of number crunching, it's often waiting for user input or an i/o request to complete. Halting the processor effectively puts it into a suspended mode, consuming less power and therefore giving off less heat. In use it often decreases heat by 10 degrees centigrade.
Modern operating systems like
Linux or
Windows NT execute the HLT instruction in an idle priority thread.
Windows 98 was supposed to have this feature but it was pulled out last minute.