>
> In the following session, MS-DOS debug is used to read and write
> using the port at 0x80. Note that what is written, will be read back.
> Therefore, it is NOT an "unused" port.
The comment is asm-i386/io.h is just wrong.
Port 0x80 is the POST diagnostic port. Since the original IBM AT, it's been used
by the BIOS during POST to indicate status and error information. Only Compaq
has used another port (0x84). If you have one of those diagnostic cards with the
7-segment display on it, this is the port it is listening to.
Whether this method is a Good Thing is open to question. On the one hand, it
just lets the hardware do whatever it wants, which eliminates the need to know
about the motherboard in use. On the other hand, it is ugly and is potentially
much slower that it need to be.
This whole issue is a consequence of not using the BIOS at all.
John