On Tue, 15 Feb 2005, Gene Heskett wrote:
> On Tuesday 15 February 2005 10:02, Eric Siegerman wrote:
> >On Tue, Feb 15, 2005 at 10:53:55AM +0100, Geert Uytterhoeven wrote:
> >> I thought[*] 7 was the highest priority, and 0 the lowest (on a
> >> narrow channel).
> >
> >That's what I recall too.
> >
> >> Wide devices have an even lower priority: 15 to 8.
> >
> >This sounds vaguely familiar too, but I'm *far* less certain
> >about it.
> 
> The bus scan at bootup starts at 0, thats true, but I'm not aware that 
> any device number has a 'higher priority'.  As far as the driver 
> scanning the bus to see who yelled when the ATN line gets pulled 
> down, I don't think its done that way at all as the ACK or whatever 
> that comes back then causes the device thats waving its hand to put 
> its address on the bus.  So there isn't even a priority there that I 
> know of.  If another device also puts up its hand, then I think it 
> gets read for the device number and otherwise queued, and will be 
> serviced as soon as the first device has been told what to do.  The 
> scsi bus multitasks very well indeed in well written code.  Every 
> device, including the controller, should have collision avoidance 
> built in since thats a basic part of the scsi specs.

A device puts its address on the bus by asserting the data line that
corresponds to its own ID. If multiple devices do that at the same time,
multiple data lines become asserted and the highest priority device wins.

For a wide bus, if none of the 8 lowest data lines are asserted, the bus master
will check the 8 highest data lines. Hence the lower priority of devices 8-15.

Gr{oetje,eeting}s,

                                                Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert AT linux-m68k 
DOT org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                                            -- Linus Torvalds