User's Guide

Page 58 DDS-4 SCSI Interface Manual, Rev. B
Command Descriptor Block Field Description
The following table defines the fields in the CDB.
Field
Name Bytes Bits Description
PC bits 2 6-7 These Page Control bits select one of four possible types of log
information. These bit settings are shown in Using Page Control Bits.
Page
Code
2 0-5 This field indicates the requested page code.
Parameter
Pointer
5-6 7-0 This two-byte field specifies the beginning parameter code to be
returned to the initiator. All remaining parameter codes are returned
in ascending order.
This field is invalid of page code 0. If the Parameter Pointer field is
nonzero when page 0 is requested, a Check Condition is returned.
The sense data are set to Illegal Request, Invalid Field in CDB.
Allocation
Length
7-8 7-0 The Allocation Length field specifies the maximum amount of
memory space (in bytes) that the initiator has reserved for log sense
data.
The drive returns the amount of bytes contained in the requested
page or the requested Allocation Length, whichever is less. If the
Allocation Length is greater than the actual page length, the transfer
is truncated.
The data for only one log sense page can be transferred with any one CDB. Log
sense data is organized by pages. The parameter data for each page is preceded
by a four-byte page header. The page header specifies the page code returned and
the length of that page in bytes.
If the Parameter Pointer field is used to specify a starting parameter code, a page
header is still returned.
Following the page header are 0 or more log parameters. Each log parameter is a
special data structure that contains several description bytes and the parameter
value itself. (Subsequent subsections describe the log parameter fields.)
The following table details the log sense page code field.
Bits
5 4 3 2 1 0 Page Page Information
0 0 0 0 0 0 0 List of available pages
0 0 0 0 1 0 2 Write error counts
0 0 0 0 1 1 3 Read error counts
1 0 1 1 1 0 2E Tape Alert Flags
1 1 0 0 0 0 30 DDS tape log (vendor unique)
1 1 0 0 0 1 31 Tape capacity (vendor unique)
1 1 1 0 0 1 39 Data compression transfer log