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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH] Implement PC port80 debug register.
Date: Mon, 29 Jun 2009 15:17:17 +0100
References: <1246262725-23825-1-git-send-email-jljusten@gmail.com>
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To: qemu-devel@nongnu.org
Cc: Jordan Justen <jljusten@gmail.com>

On Monday 29 June 2009, Anthony Liguori wrote:
> Jordan Justen wrote:
> > From: jljusten <jljusten@jljusten-laptop.(none)>
> >
> > In PC systems, the byte I/O port 0x80 is commonly made into a
> > read/write byte.  BIOS and/or system software will often use
> > it as a simple checkpoint marker.
>
> What software does this?  Typically, port80 is used as an IO delay
> mechanism.  I'm not aware of it being used to read/write arbitrary data.

Port 0x80 is generally the bios debug port. It's not uncommon for motherboards 
to display the values written to this port via an onboard 7-segment LED.

Paul