Contigent Allegiance question

Lohmeyer, John JLOHMEYE at
Tue Aug 15 14:12:00 PDT 1995

Hello Steve.  I've imbedded my response to your CA questions, below -- John

Steve Gaskill wrote:
>1. Which of the SCSI-3 documents describes the Contingent Allegiance
>I snooped through SIP, SPI, SPC -- but couldn't find it.  SBC maybe?

It should be in SCSI-3 Architecture Model (SAM), although CA and ECA are 
being replaced by Auto Contingent Allegiance (ACA), which makes several 

>2. Is a SCSI-2 target that supports tagged command queuing and CA (but not
>permitted to enqueue commands from other initiators?  Or _must_ it respond
>BUSY status until the CA condition is cleared?

SCSI-2 requires BUSY status to be returned to other initiators while a CA or 
ECA condition exists.  The word "shall" is legalese for mandatory.  You 
found the pertinent sentence:

>in section 7.7, I find "While the extended contingent allegiance condition
>exists, the target shall respond to any other request for accessto the
>unit from another initiator with BUSY status."

>Does the lack of the word "shall" in the first quote mean that the target
>not have to respond with BUSY status, and can accept more commands
>to be placed into the frozen queue)?

No, quite the opposite.  "Shall" means the behavior is mandatory, not 

>I have a SCSI book that states the target may accept (enqueue) commands 
>other initiators, but it's not so clear to me from the actual spec.

If the book says that commands may be enqueued during a CA or ECA condition, 
then it is wrong.  Although, I suspect that for most error conditions on 
disk drives, such an implementation might actually work fine.  However, many 
error conditions on sequential-access devices would lead to horrible results 
if the device does not lock out other initiators while the error is being 
recovered.  This is particularly true for error conditions that cause an 


John Lohmeyer             E-Mail:  john.lohmeyer at
Symbios Logic Inc.         Voice:  719-573-3362
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