🚀 go-pugleaf

wells@cdsmn.mn.org (Rich Wells) writes:

>In article <1992Jul30.115954.12123@donau.et.tudelft.nl> cornet@zen.et.tudelft.nl (Jan-Pieter Cornet) writes:
>>
>>On a 286, the fastest way to get out of protected mode is to cause a triple
>>fault resulting in a shutdown, which (non-buggy ;) external circuitry
>>converts to a reset almost immediately (the keyboard chip takes quite a
>>while to perform the reset, that's why the HLT; JMP loop is there, and
>>that's why this method is faster).
>>
>>You can cause a triple fault by lowering the IDT limit to 0, and cause
>>an exception, for example by doing MOV AX,[FFFF].

>Is this the technique that Microsoft used for the original version of
>OS/2?  I remember reading that they had some really neat way to re-enter
>real mode from protected mode, but they wouldn't divulge the details
>at the time.

I really have no idea... but I forgot to mention, an even quicker (_much_
quicker because no reset and BIOS POST are involved) and also much much
dirtier way to return to real mode from protected mode is to use the
undocumented LOADALL instruction on the 286. I haven't actually used this,
but it is possible. See the file <msdos.sysutl>vu-xm1c.zip on simtel
or mirrors.

Knowing that microsoft knew about this instruction (they use it in HIMEM.SYS)
it is very well possible that they also used this in OS/2 1.x.

-- Jan-Pieter
cornet@duteca.et.tudelft.nl