Mike Long writes:
> 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.

My question for everyone is: why is the A20 enabler code put so late
in the boot blocks? Can it be done first thing if desired?

Perry