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From: ballou@maypo (Kenneth R. Ballou)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Switching from Protected to Real Mode
Message-ID: <1203@cartan.Berkeley.EDU>
Date: Thu, 27-Aug-87 04:23:48 EDT
Article-I.D.: cartan.1203
Posted: Thu Aug 27 04:23:48 1987
Date-Received: Sat, 12-Sep-87 10:07:22 EDT
References: <1387@imagen.UUCP> <4047@utai.UUCP> <1343@gryphon.CTS.COM>
Sender: news@cartan.Berkeley.EDU
Reply-To: ballou@bosco.berkeley.edu (Kenneth R. Ballou)
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In article <1343@gryphon.CTS.COM> greg@gryphon.CTS.COM (Greg Laskin) writes:
>In article <4047@utai.UUCP> anton@ai.UUCP (Anton Geshelin) writes:
>>There is an undocumented instruction in the 286 which will throw you back
>>into the unprotected mode.  Microsoft has found it so useful that they
>>insist that Intel not fix it in the 386.  At least, these are the
>>rumors I heard.

	Well, I would be suspicious of this rumor insofar as one can disable
protected mode quite readily on the 80386 without having to resort to
resetting the CPU.  The procedure for doing so is clearly documented in
Section 14.5 of the 80386 Programmer's Reference Manual, titled "Switching
back to real-address mode."  The upshot is that the Protected Mode bit in
the system register CR0 (akin to the MSW of the 80286) is not sticky.

>I saw a real posting in comp.sys.intel a few months ago that made obscure
>reference to such an instruction, LOADALL or thereabouts.
>
>The poster, perportedly an Intel person speaking on his own, stated:
>   1)  The documentation for this undocumented instruction was made
>	made available to Intel largest customers (IBM and Microsoft)
>	under non-disclosure agreements.

Grumble!  Speaking only for myself:

>>> FLAME ON! <<<
If this is true, then this is the sort of absurd information hoarding that
benefits no one except whoever makes a tidy profit from a monopoly of know-
ledge.  Of course, I still haven't forgiven Intel (not that they care, I'm
sure) for putting the global/local and privilege level bits in the *LOW
THREE BITS* of the segment descriptor.  I would be very impressed if anyone
could give me a convincing explanation that there is enough gained by this
in return for sacrificing a 29-bit linear address space.

Perhaps I will be accused of participating in the seemingly popular sport of
Intel bashing.  However, I think my comment (about segment descriptor
structure) is reasonable enough, and frankly I don't feel guilty about minor
bashing of people/companies that hoard information such as "undocumented
instructions" from all but the wealthiest.  (Have I been listening to
Richard Stallman too much?  :-) :-) :-) :-) :-)  )

-------------------------
Kenneth R. Ballou	ballou@bosco.berkeley.edu