Article View: comp.os.msdos.programmer
Article #2441Enabling the A20 line (was Re: point to an address above 1M )
From: Mark Grand
Date: Sun, 30 Sep 1990 14:23
Date: Sun, 30 Sep 1990 14:23
25 lines
1544 bytes
1544 bytes
In article <10850001@hp-ptp.HP.COM> marcb@hp-ptp.HP.COM (Marc Brandis) writes:
is just ignored, so that all addresses still lie in the first megabyte.
However, the 80286, 80386 and 80486 have more than 21 address lines,
and there the 21st address bit is set in this case, causing the CPU
to access memory above the first megabyte. For compatibility reasons,
all systems based on the 286, 386 and 486 include special circuitry,
that causes this address line (A20) to be zero. This "feature" can
be disabled, so that nearly 64k more memory becomes addressable in
real mode.
How does one go about enabling the A20 line?
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Mark Grand
Premenos Corporation 415-602-2073
1000 Burnett Avenue, Suite 200
Concord, CA 94520 ...!attmail!premenos
...!{sun, ames, att}!pacbell!premenos!mark
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Mark Grand
Premenos Corporation 415-602-2073
Technical Details
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Path: arc1-ca.pugleaf.net!utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!usc!sdd.hp.com!mips!pacbell.com!pacbell!premenos!premenos!mark From: mark@premenos.premenos.COM (Mark Grand) Newsgroups: comp.os.msdos.programmer Subject: Enabling the A20 line (was Re: point to an address above 1M ) Message-ID: <MARK.90Sep30072325@premenos.premenos.COM> Date: 30 Sep 90 14:23:25 GMT References: <14016@hydra.gatech.EDU> <10850001@hp-ptp.HP.COM> Sender: mark@premenos.uucp (Mark Grand) Organization: Premenos Lines: 25 In-Reply-To: marcb@hp-ptp.HP.COM's message of 26 Sep 90 16:13:06 GMT