Re: IO delay, port 0x80, and BIOS POST codes

[Richard B. Johnson]

On Thu, 14 Mar 2002, Linus Torvalds wrote:

> 
> On Thu, 14 Mar 2002, Richard B. Johnson wrote:
> > 
> > Well I can see why he's an EX-Phoenix BIOS developer. A port at 0xed
> > does not exist on any standard or known non-standard Intel/PC/AT 
> > compatible.
> 
> Note that "doesn't exist" is actually a _bonus_. It means that no 
> controller will answer to it, which causes the IO to time out, which on a 
> regular ISA bus will also take the same 1us. Which is what we want.
> 
> Real ports with real controllers can be faster - they could, for example,
> be fast motherboard PCI ports and be positively decoded and be faster than
> 1us.
> 
> 		Linus
> 

Well no, IO doesn't "time-out". The PC/AT/ISA bus is asychronous, it's
not clocked. If there's no hardware activity as a result of the write
to nowhere, it's just a no-op. The CPU isn't slowed down at all. It's
just some bits that got flung out on the bus with no feed-back at all.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

                 Windows-2000/Professional isn't.