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

[Jordan Justen]

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 <address@hidden> wrote:

On 06/29/2009 04:47 PM, Anthony Liguori wrote:

Jordan Justen wrote:

From: jljusten <address@hidden(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.

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