Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!ssc-vax!bcsaic!paula
From: paula@bcsaic.UUCP (Paul Allen)
Newsgroups: comp.sys.intel
Subject: Re: 386 "real" mode access beyond 1M using data_32 instruction prefix
Keywords: 80386,real mode
Message-ID: <10121@bcsaic.UUCP>
Date: 15 Feb 89 00:12:02 GMT
References: <529@h-three.UUCP>
Reply-To: paula@bcsaic.UUCP (Paul Allen)
Organization: Boeing Computer Services AI Center, Seattle
Lines: 30

In article <529@h-three.UUCP> tony@h-three.UUCP (me) writes:
>I need to write a routine for a 386 in "real" mode (actually a firm-
>ware application) that can read/write data above 1M.  At first glance,
>the instruction and address prefixes (0x66, 0x67) would seem to be
>the way to do this; but I can't get them to work, and even have a 
>suspicion they won't work.

	[assembly code that gets General Protection Fault]

>Is this the wrong way to do this?
>
>Is it even possible to access above 1M without going into full protected
>mode?

I was curious about this too, so I walked over to our library and
checked a 386 reference.  It turns out that the answer is both yes and
no.  In real mode, the offset part of an address is always treated as
16 bits, regardless of any 32-bit prefixes you specify.  The highest
address you can reach in real mode without triggering a General
Protection Fault is FFFF:FFFF, which is almost 64K more than 1Mb.  The
386 does not wrap around at the 1Mb boundary like the 8086 does.  In
general, you must switch to protected mode in order to use memory above
1Mb.

Paul Allen

-- 
------------------------------------------------------------------------
Paul L. Allen                       | pallen@atc.boeing.com
Boeing Advanced Technology Center   | ...!uw-beaver!ssc-vax!bcsaic!pallen