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
4-5
Structure of the WRITEREAD Buffer
Structure of the WRITEREAD Buffer
The buffer passed to and returned from the WRITEREAD procedure contains a header
block, a control field whose structure corresponds to that of a CAM control block
(CCB), an optional sense-buffer field, and an optional data-buffer field. Figure 4-1
shows the basic layout of the buffer.
Note the following rules for formatting the buffer:
•
The header block must be the first field in the buffer. For details about the header
block, see The WRITEREAD Buffer Header Block on page 4-6.
•
The second field must be the CCB. For details about the CCB, see The CAM
Control Block (CCB) on page 4-9.
•
The sense-buffer and data-buffer fields are included if required by the specified
operation.
The existence, location, and length of the sense buffer and data buffer are specified by
fields within the CCB when the specified operation is the CAM operation Execute SCSI
I/O.
The sense buffer is optional, but it should usually be present so that sense information
can be returned when a check condition occurs on the device.
The data buffer must be present when a SCSI operation transfers data to or from the
device.
In normal operation, an application or driver sets up the header block and CCB, making
provision for any data that is to be transferred in either direction. The WRITEREAD
operation transfers the buffer to the IOP, which executes the operation, updates the CCB,
fills in the sense buffer and data buffer as needed, and returns as much of the buffer to
the application as the read-count parameter permits (see Figure 4-2
and Figure 4-3).
To minimize the number of empty bytes copied through the message system with each
WRITEREAD operation, set the read-count and write-count parameters of
WRITEREAD so that empty buffers are not transferred. Figure 4-2
shows an example of
the read count and write count for an operation that reads data from a device. Note that,
in this example, the amount of data written to the IOP contains only the header block
and CCB; however, the application reads back the entire buffer, which contains the
header block, the updated CCB, and both the sense buffer and data buffer, which now
contain information.
Figure 4-1. WRITEREAD Buffer Structure
Header
Block
CCB
Sense Buffer
(optional)
Data Buffer
(optional)
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