[OpenBIOS] Why protected mode?

[Timothy J. Massey]

On Thu, 15 Jul 1999 08:39:45 -0400, Andrew Kohlsmith wrote:

>> > That was only on the '286, which required a processor reset to get back
>> > into real mode and was assisted by the keyboard controller, one of the
>> > Great PC Hacks.  It is not as traumatic with '386 and beyond.
>
>I also wanted to add that all this nonsense was necessary because int he
>design of the 80286 there was a method of ENTERing protected mode, but no
>clean way to get back.  I'm curious to know whether the double/triple fault
>was used as a way back because that's how Intel had *designed* it, or if the
>lack of method to come back to Real mode was an oversight.
>
>Looking at pages like x86.org, I tend to think the latter.

Definitely a mistake.  Well, let's say shortsighted thinking from Intel. 
They thought, "Once in protected mode, who would want to go back to real
mode?"  In fact, that was one big reason why OS/2 version 1.x was slow and
unreliable when it came to DOS programs.  Resetting the processor to real
mode on the 286 had been described by Intel as "Putting a car's transmission
into reverse at 50 miles per hour".  (That and the lack of hardware
virtualization via the IO map...)

Of course, unless we're writing a bios for 286 computers, that's all a moot
point!  :)  But an interesting piece of trivia nonetheless.

Tim Massey


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