20 Mar
2012
20 Mar
'12
1:49 a.m.
On 19 Mar 2012 at 17:20, Eric Smith wrote:
The processor architects at Intel assumed that once
you entered
protected mode, you wouldn't want to go back. Arguably that was a
little short-sighted of them, but then, back when the 286 was being
designed (starting around 1980) the IBM PC wasn't even on the market,
so it clearly wasn't obvious that the IBM PC and its descendants were
going to become the most popular general-purpose computing platform on
the planet.
I think it was reasonable--how many mini- and mainframe machines are
*always* operated in protected mode, while "real" mode is viewed as
sort of a bootup or maintenance mode? Aside from maintenance tasks,
would you really expect to see a S/370 with the DAT box disabled in
normal everyday use?
To their credit, both IBM and Microsoft tried to make 286 PM work for
real-mode programs. But programs from the x86 that fooled with the
segment registers really messed that idea up.
I can't really fault Intel for their thinking. 286-style
segmentation, yes--that was just awful. But that was corrected
with the 386. But then, by the time of the 386, who needed an A20
gate?
Perhaps, if on the release of the PC, a "design guide" would have
been published that anticipated the 286 PM release might have been
useful.
But for how many years was the 80386 usually run in real mode?
--Chuck