Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: 4 gigs in Real Mode (Works!) > Why A20?.
Message-ID: <25c97ae8@ralf>
Date: 2 Feb 90 11:19:52 GMT
Sender: ralf@b.gp.cs.cmu.edu
Organization: Carnegie Mellon University School of Computer Science
Lines: 27
In-Reply-To: <3288@plains.UUCP>

In article <3288@plains.UUCP>, harlow@plains.UUCP (Jay B. Harlow) wrote:
}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. 
}>
}  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 

Probably because a Certain OS Company actually *used* the 1M wraparound....
(the CP/M-compatible "CALL 5" entry point jumps to 000C0h, but is
constrained on HOW it jumps there by the need for PSP:0006h to be the
size of the first segment. That's how CP/M programs determined how much
space they had, since the jump instruction at location 5 on a CP/M
machine goes to the lowest byte of the OS, which is the next byte after
the last byte available to the program)

--
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"How to Prove It" by Dana Angluin              Disclaimer? I claimed something?
14. proof by importance:
    A large body of useful consequences all follow from the proposition in
    question.