SCC Addressing

George Penokie GOP at
Tue Jun 25 07:16:44 PDT 1996

* From the SCSI Reflector, posted by:
* "George Penokie" <GOP at RCHVMP3.VNET.IBM.COM>
 Ron writes,
>Can any of the addressable LUNs reported overlap?  In other words, if
>there are some devices in the array that have been configured into a
>volume can the REPORT LUNs report back both the peripheral devices and
>volume set associated with this storage?  If so, how does the initiator
>(host) determine that this is the case?
 Yes addressable LUNs of different types (eg volume sets and peripheral
 devices) may overlap.  This would be a normal behavior as volume sets are
 just a collection of peripheral devices.  In a SCC device that support 2
 byte addressing you tell which type of device is being addressed by
 decoding the ADDRESS METHOD field in the two byte address.  That field
 will indicate if the address is for a volume set or a peripheral device.

 In a SCC device that only supports 6 bit LUN addressing (parallel
 interconnect) the ADDRESS METHOD filed would contain 00b, the BUS NUMBER
 field would contain 000000b and the TARGET/LUN field would contain the 6
 bit LUN address (bits 6 and 7 would be 00b) then you would look at bit 5
 of the TARGET/LUN field:  if it is set the address is for a volume, if
 cleared the address is for a peripheral device.

 How you deal with a parallel SCC device that supports both 2 byte and 6
 bit LUN addressing is not stated anywhere. One way to do it would be to
 return both the 2 byte and 6 bit addresses for each LUN and let the
 application client figure out that there are duplicate addresses for the
 same device.  It can find this out by checking the universal identifier in
 the Inquiry command. Any comments??

>How does one address a peripheral device?  The logical unit and addressing
>and peripheral device addressing methods seem to accomplish the same thing
>of providing a way to do bus/target/LUN addressing.  What is the real
>difference in these two addressing methods and when should each be used?

 You address a peripheral device by sending a command to the SCC target
 with the LUN set to the address of the peripheral device.  If you have a
 SCC device that is using 2 byte addressing the ADDRESS METHOD field is set
 to either 00b or 10b.  If you have a SCC device that is using 6 bit
 addressing the VOLSEL bit would be set to 0b.

 The reason for having two address methods for 2 byte addressing is because
 deferent environments have different requirements on the numbers of buses,
 targets, and LUNs that can be attached to the SCC device.  One method
 gives a high number of targets(255 max) and buses(63 max) and assumes all
 the targets only have one LUN each.  The other method allows for targets
 that have multiple LUNs(32 max) but reduces the number of buses(8 max) and
 targets(64 max).

 Which one you use depends on your requirements for the number of LUNs that
 the target devices within your SCC device contain.  If you only have
 standard hard drives that contain only LUN 0 then the 00b method would be
 a good choice.  If, however, you envision targets with multiple LUNs being
 attached then the 10b method would be a good choice.  Although there is
 nothing to prevent you from using the 00b method on some devices and the
 10b method on others within the same SCC device.

 Bye for now,
 George Penokie

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