Terminate Immediate Sequence                                   X3T9.2/88-158R0

  TO:  John Lohmeyer

FROM:  Steve Goldman

DATE:  December 5, 1988

The terminate immediate sequence involves the transmission of two special 
messages and is intended to give initiators an increased degree of real-time 
control when communicating with SCSI target devices.  Specifically, READ or 
WRITE commands which involve long transfer lengths may be terminated before 
the transfer count has been exhausted without requiring the entire transfer to 
be repeated later as with the abort message, i.e. blocks already successfully 
read from or written to the medium need not be re-transferred.  This 
facilitates initiators which cache data on the host computer or host adapter 
to initiator long background read or write operations while still providing a 
means of terminating the operation before all data has been read or written in 
order to initiate a foreground read or write operation due to a cache miss or 
an FUA read or write.

TERMINATE IMMEDIATE
This message is sent from the initiator to the target during a read or write 
operation to indicate that the current I/O process should be terminated as 
expeditiously as possible.  Upon receiving this message the target may 
continue transferring data but should reduce the transfer length to a value 
which allows the current command to be terminated as soon as possible.  After 
the last byte of the data phase for that command the target shall switch to 
message in phase and send the REPORT TRANSFER LENGTH message. The target shall 
then switch to the STATUS phase and terminate the command in the normal way.   
The status byte shall indicate the success or failure of the operation for the 
blocks written to the medium or transferred to the initiator.


REPORT TRANSFER LENGTH
This message is sent by the target for commands which have been terminated 
due to a TERMINATE IMMEDIATE message in order to indicate to the initiator the 
actual number of logical blocks written to the medium in the case of a write 
operation, or transferred to the initiator in the case of a read operation.  
After sending this message, the target shall then switch to the STATUS phase 
and terminate the command in the normal way.   The status byte shall indicate 
the success or failure of the operation for the blocks written to the medium 
or transferred to the initiator.