Why Windows 95 left HLT on the cutting-room floor

"Microsoft vet Raymond Chen first told the story of HLT and Windows 95 more than 20 years ago. The instruction tells the CPU to effectively shut itself down until the next hardware interrupt - ideal for laptops, since power consumption would be hugely reduced. Microsoft had it implemented and working in Windows 95, but found that there were some devices out there, including from a "major manufacturer," that would be locked up unrecoverably by the instruction. Not wanting to be blamed, Microsoft pulled it."

""Windows 95," said Chen, "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.""

"So why didn't Microsoft just special-case devices that would be bricked by the HLT instruction in Windows 95? Chen has finally explained. The answer is simple - Microsoft wasn't sure how many susceptible devices were already out in the wild, and so opted to leave it out rather than take the risk. Users of modern Windows are accustomed to Microsoft erecting compatibility barriers to prevent an operating system update from being installed if there is something about the hardware"

Microsoft implemented the HLT instruction in Windows 95 to reduce power consumption by halting the CPU until the next hardware interrupt. Some devices, including from a major manufacturer, would lock up unrecoverably when HLT executed, creating an unusable system. Microsoft judged that too many systems were affected to safely ship detection-only protections, so HLT was removed to avoid bricking customer machines. The widespread nature of the issue suggested fragmented device behavior, and removal of HLT led to third-party utilities reintroducing the functionality for users willing to accept the risk.

Read at Theregister