Hi Andy,

 ET> Of course you can still boot from SCSI-ID 0 or 1, but it's not
 ET> restricted any more to boot from those two

 AR> Alright granted.  But remember way back years ago when 
 AR> CPUs were slow and it was important to put your modem on 
 AR> a low IRQ. Think about the reason for that.  Sure it would 
 AR> work on most any IRQ, but not as well under all conditions. 

The issue with a low numbered IRQ for a UART revolved around the importance of 
clearing the Received Data Buffer before another character arrived. A Serial
port is not a fast device, however it was important to service its interrupts
quickly. Buffered UARTS resolved a lot of these issues but its still an area
that needs to tbe considered for response time critical applications such as
FAX decoding in the PC, not the modem.

 AR> But can you give me a good reason NOT to put my most demanding and 
 AR> fastest SCSI devices on the lowest SCSI ID and the next 
 AR> fastest and next most demanding devices in order of higher SCSI IDs 
 AR> with the slowest and least demanding devices on the highest SCSI ID, 

It all comes down to the type of device being serviced. For a long time I used 
an Archive Viper SCSI Tape drive that had minimal buffering. To optimise
performance it was ESSENTIAL to keep providing it with data so that it would
stream and not pause. Because the HD's were so much quicker than the Tape
drive, I kept the Tape at a higher SCSI address than all the HD's (higher
addresses are serviced BEFORE lower ones). When it was at a LOWER address than 
the HD's, the intensive system activity that used the HD's a lot could
frequently stop the Tape drive from streaming. 

To optimise performance for ALL devices on a common bus, you should set it up
to service the most response critical device FIRST, followed by the rest of
the devices in descending order of need for service. A Fast device with large
buffers (such as a modern HD) can generally afford to wait a bit longer than a 
slow device with minimal buffering that needs a response NOW. With only 1 or 2 
devices (not including the Host Adapter) on a SCSI bus the addressing issue is 
probably a moot point.

Regards...........pk.


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