The 80386 HLT instruction tells the CPU to stop executing instructions until a hardware interrupt occurs. This was traditionally the instruction performed by the operating system when it had no work to do, allowing the CPU to go into a low-power mode. (In modern times, operating systems use other instructions like MWAIT to go into even lower power modes when they have nothing to do, but those fancy instructions didn’t exist in 1995.)

A while ago, I discussed how Windows 95 intentionally did not use the HLT instruction when idle because there were multiple models of laptop which locked up unrecoverably when the CPU executed a HLT instruction. People wrote aftermarket components that performed HLT instructions on behalf of Windows 95 to get low-CPU behavior, with the added notation “I can’t believe Windows 95 is so stupid that it doesn’t have this feature built-in.”

I guess these people never got bug reports from customers saying “I installed your custom program, and now my laptop freezes up as soon as it boots. How do I uninstall it if my laptop freezes up at boot?”

And that’s the problem: The failure mode is a brick.

Windows 95 could have added detection for the systems that froze up on HLT instructions, but since there were many such systems, the risk was that by the time Windows 95 shipped, not all affected systems would have been identified. The fact that many systems were affected means that this was not an isolated case. We’re probably seeing the tip of a very fragmented iceberg.

Since the failure mode is a system that is unusable, the cost of a false negative was far too high. We just had to remove the HLT.