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Date: Mon, 29 Jun 2009 16:34:35 +0200
From: Chris Lalancette <clalance@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Implement PC port80 debug register.
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To: Avi Kivity <avi@redhat.com>
Cc: Jordan Justen <jljusten@gmail.com>, qemu-devel@nongnu.org

Avi Kivity wrote:
> On 06/29/2009 04:47 PM, 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.
>>
> 
> It's often used in BIOS code.  There used to be seven-segment cards 

s/used to be/are/

http://www.biosman.com/port80.htm

(although only for ISA/PCI slots, which may be hard to find on modern motherboards)

> you'd plug into a computer that would show you port 80 in real time.  I 
> think it's a write-only port, though.

-- 
Chris Lalancette