Subject: Re: Another changer, another changer problem
To: None <>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: current-users
Date: 10/14/1998 08:37:11
>>> As I showed above, the only reasonable and reliable way to handle
>>> cards is by the physical slot that it is in.

But many busses don't permit detection of physical slot.  Or are you
proposing a redesign that makes the system unusable on such machines?
You'd desupport a lot of our present machines by such a move.  (VME and
Qbus/UNIbus come to mind immediately; based on my minimal understanding
of it, I think ISA is in the same boat.)

Heck, even SCSI doesn't do things in physical order.  You can't tell,
from software, whether that disk at ID 4 is first, last, or somewhere
else in physical order on the chain.  And if you're willing to look at
ID switches or jumpers, I submit that's exactly the same thing as VME
or Qbus address switches or jumpers - the only differences are (1) on
SCSI they're often (not always!) visible without having to open any
enclosures and (2) the address space is much smaller (8 or 16 possible
addresses instead of 64K or more possible addresses).

>>> Having switches of some sort only leads to limitations of how many
>>> items of a given kind you can put in the machine and various other
>>> conflicts.
>> Hmmm... I've never seen much of a problem with using address
>> switches and such on VME systems.

(Me neither.)

> Although, I've administrated VME systems, I haven't had to do much
> with the hardware.  Do they let you select from amongst a large set
> of addresses, or can certain cards only appear at a small limited set
> of addresses?

In general, the former.  It's certainly possible to build a card with
much of the addressing logic hardwired (one Qbus card in particular is
somewhat infamous for having *one* address switch, which selects
between two hardwired addresses).  Most cards, though, have a bank of 8
or 16 or 24 or whatever switches, which allow you to put the card more
or less wherever you want in the bus address space.  (Often the low few
bits will be forced to zero, which is not a surprise since the card's
presence in the address space is not just a single address, but usually
a handful of consecutive addresses, sometimes more than that.)

					der Mouse

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