Category Archives: 286

SGDT/SIDT Fiction and Reality

PSA: Actual hardware behavior takes precedence over vendor documentation. Or, as they say… trust but verify. A reader recently complained how Intel and AMD do not implement the SGDT and SIDT instructions the same way. AMD documentation states that these … Continue reading

Posted in 286, 386, AMD, Documentation, Intel | 37 Comments

There’s More to the 286 XENIX Story

It turns out that there is a rather interesting story behind the 286 XENIX incompatibility with 386 and later processors. Here’s roughly what happened in chronological order. In 1982, Intel released the iAPX 286 processor, later known as the 80286 or simply 286. This … Continue reading

Posted in 286, 386, Intel, Microsoft | 39 Comments

IBM XENIX: Two Steps Forward

There are reasons to revisit an old topic. Very old, considering that IBM Xenix 1.0 was released in 1984, well over 30 years ago. To recap, this version of Xenix is unique in that it runs only on 286 processors. It … Continue reading

Posted in 286, IBM, VirtualBox, Xenix | 13 Comments

IBM XENIX 1.0 Incompatibility Details

Some time ago I wrote about IBM PC XENIX 1.0 and why it won’t work on 386 and later processors. Thanks to a kind reader, I’ve been able to analyze the object files used to link the kernel, and I believe … Continue reading

Posted in 286, 386, Microsoft, Xenix | 19 Comments

Better Late Than Never

Better late than never, although in this instance, it’s really really late—about thirty years late. In the world of computing, that is eternity. The talk is about the new CR4.UMIP control bit documented in the latest (revision 58) Intel SDM, … Continue reading

Posted in 286, Intel, x86 | 10 Comments

Basement Finds

The other day I attacked an old and long forgotten 286 PC stashed away in the basement. The PC is dead because the power supply blew years ago and the motherboard has a non-standard power connector. But the case was … Continue reading

Posted in 286, ATi, PC hardware | 10 Comments

IBM XENIX: One Step Forward, One Step Back

A while ago I wrote about why IBM PC XENIX 1.0 can’t work on any CPU other than a 286. But not content to leave well enough alone, I attempted to patch this version of XENIX so that it would … Continue reading

Posted in 286, 386, IBM, VirtualBox, Xenix | 5 Comments

Forward Compatibility, Landmines

Several years ago, after attempting to get a very old 286 version of Xenix running in a VM, I concluded that it was probably incompatible with any 386 and later processor. Recently I revisited this issue and examined the problem … Continue reading

Posted in 286, Intel, Xenix | 29 Comments