How can the address line 20 be enabled

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#3403

From: Adam Craig Seychell
Date: Mon, 28 Sep 1992 08:02

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          How can the address line 20 be enabled

The a20 line must be enabled for the processor ( only in
protected mode ) to use all of the extended
memory and not just the even megabyte blocks.
I know that the keyboard controller does this and that a
single bit in
a single register must be set to enable the a20 line.
But setting this bit is not as easy as using the OUT
instruction but 
rather a instruction must be sent to the keyboard controller
then
a few other security registers must be played with and only
then
the A20 line can be enabled. As you can see this bit is not
easy to get too.
one would think that loading HIMEM.SYS will enable it because of the
message it gives.
	"A20 line enabled  " 
but it doesn't seem to stay enabled all the time, only when HIMEM.SYS needs to.
Because when I try to access any memory from 100000h to 1fffffh it
accesses the low megabyte of memory.



     Does any one out there know how this can be done.
     even the smallest information about the A20 line will be
appreciated

          email to s921880@minyos.xx.rmit.oz.au

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From: s921880@minyos.xx.rmit.oz.au (Adam Craig Seychell)
Newsgroups: comp.os.msdos.programmer
Subject: How can the address line 20 be enabled
Message-ID: <s921880.717667372@minyos.xx.rmit.OZ.AU>
Date: 28 Sep 92 08:02:52 GMT
Organization: RMIT Computer Centre
Lines: 31
NNTP-Posting-Host: minyos.xx.rmit.oz.au