Hi Peter,

You wrote to Andy Roberts:

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

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

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

Not suprisingly the designers of OS/2 considered it. As a result COM2
(IRQ3) has the highest priority on the system. OS/2 IRQ priority is:
3,4,5,6,7,0,1,8,9/2,10,11,12,13,14,15

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

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

Which is to be expected, of course :-)

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

I think you and Andy are saying th same thing in different ways.

George
___
 X SLMR 2.1a X Study the past, if you would divine the future.

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