Re: [Qemu-devel] [PATCH] Implement PC port80 debug register.

[Jordan Justen <jljusten@gmail.com>]

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Avi,

Well, I am not sure if this it globally the case for PC motherboards, but in
my experience, it has been read/write.

At least for a system such as qemu, it make it difficult to use the port80
checkpoint of software without being able to read the last value written.

-Jordan

On Mon, Jun 29, 2009 at 7:23 AM, Avi Kivity <avi@redhat.com> 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 you'd
> plug into a computer that would show you port 80 in real time.  I think it's
> a write-only port, though.
>
> --
> error compiling committee.c: too many arguments to function
>
>

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