Open SCSI Integrator's Manual for NonStop S-Series Servers

The Open SCSI I/O Process (IOP)
Open SCSI Integrator’s Manual for NonStop Servers—422988-002
4-7
The WRITEREAD Buffer Header Block
The fields of the header block contain the following information:
hdr_begin
This four-byte field contains a specific ASCII value that indicates to the IOP that the
buffer contains a CCB to control the SCSI device. This value must be set to “PASS”
in ASCII (that is, “0h50415353”), so that it can easily be found when you
investigate problems.
hdr_size
This two-byte field contains the size of the header block in bytes. This value must be
set to 20.
version
This two-byte field contains a version number used by the IOP and clients to
coordinate changes to the header block structure. The value of this field is initially
set to 1, and it is incremented each time the header block structure changes. These
changes are restricted to using the spare fields or adding an additional field in front
of the hdr_end field. It is intended that existing fields will not be moved or
redefined.
buffer_status
This two-byte field is set to a nonzero value if either of the following is true:
•
The IOP does not support the specified version of the header block.
•
The IOP detects an error in the organization and layout of the buffer or in those
contents of the buffer that can be checked before the CCB is executed.
Figure 4-4. WRITEREAD Buffer Header Block Structure
hdr_begin
("PASS")
hdr_size version buffer_status ccb_offset spare_1 spare_2 hdr_end
("THRU")
Header
Block
CCB Sense Buffer
(optional)
Data Buffer
(optional)
009CDT .CDD