Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!ucbvax!decwrl!ogicse!plains!harlow
From: harlow@plains.UUCP (Jay B. Harlow)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 4 gigs in Real Mode (Works!) > Why A20?.
Summary: Use of Gating A20....
Message-ID: <3288@plains.UUCP>
Date: 1 Feb 90 20:31:20 GMT
References: <33787.25c057cf@kontu.utu.fi> <40970040@hpindda.HP.COM>
Reply-To: harlow@plains.UUCP (Jay B. Harlow)
Organization: North Dakota State University, Fargo
Lines: 34

In article <40970040@hpindda.HP.COM> kmont@hpindda.HP.COM (Kevin Montgomery)
writes:
>> This is because your computer most probably does not decode address line
>> A20 (at least in real mode). If you want to access the addresses beyond
>> 1M you got to turn A20 ON implicitly. 
>
>Ahhhhhg!  I must be real confused- of what purpose is ignoring A20? 
  Certain Companies when they released thier NEW 286 Intel machine, for
a follow up to thier popular PC machine, decided they NEEDED to 
mimick it in EVERY aspect. Including the 1M limit wrap around
( i.e. address FFFF:0010 wraps to 0000:0000 ) at least being 'smart' and 
realizing that even 1M chunks (2nd M, 4th M, 6th M.... ) would be nice to
use in any upcoming PROTECTED mode systems they set up a special port
to toggle how the hardware sees A20, ( interesting enough the 486 has
direct support for this 'feature' )  Of course this same company chose
to ignore Intel warning about using INT 0 - INT 1F, any one try using
the BOUND inst on a AT ;-).

> What boxes do this?  
Nearly all of them, Must follow the leader ;-). who cares if it
is brain-dead..... ( from a reliable system point of view )
>


Of course ALL the clone makers, have to follow the leader to 
maintain compatibility ( or do they...... ;-)

			Jay

-- 
		Jay B. Harlow	<harlow@plains.nodak.edu>
	uunet!plains!harlow (UUCP)	harlow@plains (Bitnet)

Of course the above is personal opinion, And has no bearing on reality...