🚀 go-pugleaf

In article <817942727snz@wookie.demon.co.uk>
           wookie@wookie.demon.co.uk "John Beardmore" writes:

> In article <817725354snz@jasper.demon.co.uk>
>            mike@jasper.demon.co.uk "Mike Tomlinson" writes:
> 
> >I don't think this is the case: the 286 has a bug that makes it 
> >possible to access to first 64k of memory above the 1Mb barrier, 
> >and this was taken advantage of to allow DOS to be loaded high. 
> 
> But a 286 can access a lot more than 64k of extended memory. 16 meg once
> out of real mode.
> 
> Anyway, that's an addressing mode, not a bug !  :)  It was quite rational
> behavior that differed from an 8088 because the '88 didn't have enough
> address lines to do it.  :)

That's not the point: the 286 has a bug which allows the first 
64k of extended memory to be accessible from *real mode* - in 
other words, DOS can be loaded high on a 286 without the need to 
switch into protected mode to access it, so it also gives a 
speed increase. Switching into protected mode is done by a 
kludge involving the keyboard controller, so it is inherently 
slow.

This in an addressing mode that would normally force DOS to load
low, because it has no concept of memory above 1Mb.

Remember that on the 386, you need Himem.sys to load DOS high, as
Himem is a extended memory manager which takes care of the 
protected mode switching. DRDOS's Hidos.sys (which was part of
the original question in this thread) is aware of the 286 bug and
will load DOS high if memory exists above 1Mb, in real mode, so
avoiding the need for Protected mode switching.

You've let NT spoil you... Get a 286 - get a life! <grin>

-- 
Mike Tomlinson