SPIN/SPOUT reason for INC_512 bit

Mike Berhan mikeb at bustrace.com
Tue Aug 19 09:51:13 PDT 2008

* From the T10 Reflector (t10 at t10.org), posted by:
* "Mike Berhan" <mikeb at bustrace.com>
I cannot find the history behind the INC_512 bit in the SPIN/SPOUT CDBs.
The definition of this bit in SPC-4 is:
"A 512 increment (INC_512) bit set to one specifies that the ALLOCATION
LENGTH field (see expresses the maximum number of bytes available
to receive data in increments of 512 bytes (e.g., a value of one means 512
bytes, two means 1 024 bytes, etc.). Pad bytes may or may not be appended to
meet this length. Pad bytes shall have a value of 00h. An INC_512 bit set to
zero specifies that the ALLOCATION LENGTH field expresses the number of
bytes to be transferred."
Since the ALLOCATION LENGTH field is 4 bytes in length, I'm not sure why
there was a need to define the INC_512 bit?  I see a variety of
specifications state that INC_512 of 1 shall always return an Invalid Field
in CDB check condition.  I also see that for those devices that use INC_512
(e.g. some IEEE-1667 devices), it doesn't really seem to be necessary to me
since the ALLOCATION LENGTH is large enough to describe the transfer.  There
must be some benefit that I'm missing.	Thank you for the clarification.
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