On 8/31/2006 at 1:40 PM Hans Franke wrote:
 Now, that would be the 5150. In that context, already
the 5170 AT
is no longer compatible. And no Pentium system I can think of ever
will be. 
Well, there's the old (1990?) MS document on what a "Windows PC" is that
might serve as a more up-to-date reference for 386-and-above class
machines.
Prior to that, there was more than one kind of PC compatibility.  Take, for
example, the NEC 9801 series.  1024-byte sectors on floppies, different
BIOS, keyboard, video and port addresses from what we're used to.  The
dominant "PC" architecture in Japan for many years.  MS officially released
MS-DOS, Windows (3.x 95 and NT) specially tailored to it.  You may yet run
across references to "DOS-V" as well as 1.3MB 3.5" floppies.
There are some who might consider the Peanut not to be "PC compatible" for
that matter.
Cheers,
Chuck