Re: [linux-kernel] Re: [PATCH] x86: provide a DMI based port 0x80I/O delay override.

[Zachary Amsden]

On Tue, 2008-01-08 at 21:19 -0800, H. Peter Anvin wrote:
> Zachary Amsden wrote:
> > 
> > BTW, it isn't ever safe to pass port 0x80 through to hardware from a
> > virtual machine; some OSes use port 0x80 as a hardware available scratch
> > register (I believe Darwin/x86 did/does this during boot).
> 
> That's funny, because there is definitely no guarantee that you get back 
> what you read (well, perhaps there is on Apple.)

According to Phoenix Technologies book "System BIOS for IBM PCs,
Compatibles and EISA Computers, 2nd Edition", the I/O port list gives

port 0080h   R/W  Extra page register (temporary storage)

Despite looking, I've never seen it documented anywhere else, but I
believe it works on just about every PC platform.  Except, apparently,
my laptop.

Zach

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