[justsomehnguy]

>> All x86 processors from the 8086 onward had the HLT instruction, but it was not used by MS-DOS prior to 6.0[2] and was not specifically designed to reduce power consumption until the release of the Intel DX4 processor in 1994. MS-DOS 6.0 provided a POWER.EXE that could be installed in CONFIG.SYS and in Microsoft's tests it saved 5%.[3]

[gattilorenz]

I stand corrected, I was under the impression that the original Pentium was the first architecture that had HLT, but maybe that was the first architecture I ran Rain on, since it had benefits (having ran Win95 on a 586, but never DOS on a 486 laptop)

[jandrese]

Idle loops are harder to implement when your system doesn't have multitasking.

[Veliladon]

Even single tasked systems like MS-DOS still had interrupts. You could HLT the processor and a keyboard interrupt could wake it straight back up and resume execution anywhere in the MS-DOS kernel. It's just that the typical TDP of a CPU back then was a couple of watts so there was literally no point in HLTing instead of busy-waiting so nobody bothered.