Hi..
 >   JV> Here's a clip from the Linux kernel. He uses port 80h, which
 >  JV> shows in a port list as being used for DMA.. Any comments?
 >  Yeah.. the list I have shows 80h as free, but again, that list is 5
 > years old. Even if it is free, since 81h is already in use by a DMA
 > controller, 80h is one I would be absolutely sure to stay away from :)
Port 80h is the POST diagnostic status port - the motherboard BIOS sends 
bytes to this port during the POST to indicate what stage it's up to. 
Technicians use this (with the help of a simple ISA card that displays the 
data bieng written to 80h) to diagnose faults with motherboards that don't 
allow the POST to complete. Most decent diagnostic software (Diags 95, 
microscope, etc) also
use port 80h as a secondary status port to help debug PC's (with, for 
example, vaulty video sections).
Port 80h has been a standard since the IBM AT (Model 5170), although only 
some PS/2's subscribe to it. All other BIOS manufacturers have been using 
port 80h for POST codes for around 10 years now.
As such, Port 80h is clearly free for use by user code. (POST codes are not 
generated/required once the PC boots), has been a defined standard for around 
10 years, and is certainly guaranteed to not be used by other hardware.
As for port 81h bieng next to port 80h, well, so what. Just because they're 
numericly next to each other does NOT mean they're going to somehow conflict. 
To suggest otherwise would be to suggest that all PC hardware must be 
designed in such as way that no I/O port is numericly next to another one -- 
clearly this is NOT the case.
    Craig
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