>Date: Fri, 22 Mar 1996 00:53:14 -0500 (EST)
>From: "Perry E. Metzger" <perry@piermont.com>

>Can someone explain to me what the mysterious gateA20() calls mean in
>the i386 boot routines?

The A20 gate is one of those little featureoids that litter the PC AT
"architecture".  When IBM was designing the PC AT, they found that
if you use a segment:offset address that add to > 1 megabyte, then the
286 accesses memory beyond 1 megabyte.  The A20 gate is a simple AND
gate that they added to disable the processor's A20 address line.
With A20 disabled the AT could mimic the 8086, which wraps around to
low memory.  On powerup, A20 is disabled; you have to issue a command
to the keyboard controller to enable it.
-- 
Mike Long <mike.long@analog.com>     <URL:http://www.shore.net/~mikel>
VLSI Design Engineer         finger mikel@shore.net for PGP public key
Analog Devices, CPD Division          CCBF225E7D3F7ECB2C8F7ABB15D9BE7B
Norwood, MA 02062 USA       (eq (opinion 'ADI) (opinion 'mike)) -> nil