SBC Clarification: Write And Verify

Lohmeyer, John JLOHMEYE at
Wed Mar 29 12:48:00 PST 1995

Peter Dougherty wrote:
>When isssuing a Write And verify Command (2Eh) with the BytChk set to zero 
>is data supposed to be transfered to the target or is this a verify
>operation only?
>A BytChk bit of one explicitly states that data is transferred from the
>It seems some devices (targets and initiators) expect to see data and some

I think SCSI-2 is fairly clear on this topic and would hope SBC also says 
the same thing (in absence of any proposal to the contrary):

"9.2.22 WRITE AND VERIFY command

The WRITE AND VERIFY command (see table 146) requests that the target
write the data transferred from the initiator to the medium and then
verify that the data is correctly written. The data is only transferred
once from the initiator to the target."
"A byte check (BytChk) bit of zero requests a medium verification to be
performed with no data comparison.  A BytChk bit of one requests a byte-
by-byte compare of data written on the medium and the data transferred
|from the initiator.  If the compare is unsuccessful for any reason, the
target shall return CHECK CONDITION status with the sense key set to

It sure looks to me that after performing the write operation, the target 
reads the data back from the media to verify whether it is written 
correctly.  If BytChk is zero, only the ECC or CRC is checked.  If BytChk is 
one, then a byte-by-byte comparison is done with the media and the data in 
the target's buffer.  (I think the statement about only transferring the 
data once may have been added in SCSI-2 -- I thought SCSI-1 was silent on 
this and permitted data to be transferred from the initiator either once or 
twice, presumably using RESTORE POINTERS in the latter case.)

John Lohmeyer             E-Mail:  john.lohmeyer at
Symbios Logic Inc.         Voice:  719-573-3362
1635 Aeroplaza Dr.           Fax:  719-573-3037
Colo Spgs, CO 80916     SCSI BBS:  719-574-0424 300--14400 baud

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