SBP-2 Data Buffer Alignment

PJohansson at PJohansson at
Wed Jul 9 20:12:02 PDT 1997

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* PJohansson at
There have been many discussions in the working group about buffer alignment
and the fact that the target has to cope with arbitrarily aligned buffers
that start on any byte boundary. The "unrestricted page table" proposal has
even extended this concept to buffer segments that fall within the middle of
a scatter/gather list.


In, which describes a normal command block, the data_descriptor field
is specified as having the same format as Figure 11. Unfortunately, Figure 11
is primarily used to illustrate pointers to SBP-2 data structures (such as
ORB's, page tables and status FIFO's) which are at a minimum quadlet aligned.

This can (and has!) left some readers with the impression that data buffers
shall be quadlet aligned. This is in error and I suggest we change governing
language in to read "The format of the data_descriptor field, when it
directly addresses a data buffer, shall be a 64-bit Serial Bus address or,
when it addresses a page table,  shall be as specified by Figure 11."

This is in a stabilized section of SBP-2 but is not a change to the technical
intent as established by the working group. Since I have to seek plenary
approval for 97-158r1 and 97-195r0 , I think I should mention this at the
same time.


Peter Johansson
Technical Editor, SBP-2

Congruent Software, Inc.
3998 Whittle Avenue
Oakland, CA  94602

(510) 531-5472
(510) 531-2942 FAX

pjohansson at

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