FreeVMS

From: Jeffrey S. Sharp <jss_at_subatomix.com>
Date: Sat Jan 5 00:06:51 2002

On Fri, 4 Jan 2002, Sean 'Captain Napalm' Conner wrote:

> The 386 and above also have a virtual 8086 mode, which allows you to
> run real mode code under a protected OS---the real mode is virtualized
> (to an extent, I don't think it's pure vitualizasion like you can get
> on the IBM 360 line).

I believe that in virtual-8086 mode, priveleged instructions (e.g. OUT)
cause a trap. The OS, having set up the appropriate trap gates, gets
control and decides what to do for these instructions.

> There is an undocumented mode---real-32 bit (where the CPU is in
> 32-bit mode, but not running protected) but that's a hack and is
> probably not recomended.

That's 'flat mode'. It's not a hack; it's actually the simplest way to
set up protected mode, and requires very little work from the programmer.
Once upon a time, I built this floppy that contained a larval kernel and,
in the boot sector, a kernel loader. The only thing my 'kernel' did was
enter protected mode and print "Hello, world" on the screen. I used flat
mode.

--
Jeffrey S. Sharp
jss_at_subatomix.com
Received on Sat Jan 05 2002 - 00:06:51 GMT

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