Port I/O

[dedndave]

http://en.wikipedia.org/wiki/Triple_fault

it was used to go from protected mode to real mode   :P

[FORTRANS]

Hi,

   I have collected a number of programs that set up a flat
mode segment register.  Here is a snippet from one of them.
It should be enough to track it down.

This file was downloaded from

Programmers Heaven
http://www.programmersheaven.com

The Internet's most complete source of free downloadable programming files,
source codes, utilities, C/C++, Java, .NET, C#, and other tools for programmers

and developers. Here you will find over 10000 files and 4500 links organized
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  Inflating: FLAT.TXT  <to console>

FLAT REAL / REAL BIG / UNREAL MODE (v1.2)

Flat Real mode, Real Big mode and UnReal mode are three names with the very same
 meaning, I will call it FLAT in this text.

   You could check the usenet groups comp.lang.asm.x86,
alt.lang.asm, or alt.os.development though you would have to
go way back in time I suppose.

Regards,

Steve

[GoneFishing]

Thank you all ,guys!
Now I need some time " to go way back in time "  ;)
There's so much to be read.

Dave: so triple fault may be useful in some cases?
http://en.wikipedia.org/wiki/Triple_fault :

However, intentionally triple-faulting the CPU was found to cause the transition to occur much faster and more cleanly, permitting multitasking operating systems to switch back and forth at high speed.

[dedndave]

80386 and newer were much better processors   :P
the 80286 was often refered to as "brain-dead"

[FORTRANS]

Hi,

   Yeah, the 80286 had real mode and 16-bit protected mode.  It
could switch from real mode to protected mode just fine.  After all
that was the whole point of the 80286, protected mode accessed
more than one megabyte of memory.  But most programs then
were real mode.  And the memory access circuit was different in
protected mode than in real mode.  And interrupt logic was also
different.  So to run most programs you had to be in real mode.
And Intel did not have a way for the 80286 to transition from
protected mode to real mode.  By making protected mode
incompatible with real mode, and not providing a way to restore
real mode, prompted the brain dead comment.  The PC/AT had
the keyboard controller reboot/reset the CPU.

   The 80386 had real mode, 16-bit protected mode, 32-bit
protected mode, and virtual 86 mode.  V86 allowed protected
mode to look like real mode to run real mode programs.  Ta da,
problems fixed, more or less.

Cheers,

Steve N.