Quantum 10K II - Manual (Page 207)

SCSI Description
5–140 Quantum Atlas 10K II Ultra160/m SCSI Hard Disk Drives
READ BUFFER Command (3Ch) (continued)
Table 5–65 READ BUFFER Command — Field Descriptions
Field Description
Mode Four modes are supported by the drive; Combined Header and Data,
Data Only, Descriptor Only, and Read Data from Echo Buffer. The
permissible values for Buffer ID, Buffer Offset, are as follows:
0000b Combined Header and Data. – In this mode, data to be transferred is
preceded by a 4-byte header that contains the buffer capacity. The
resulting descriptor and data are transferred to the initiator during the
command’s data-in buffer transfer.
Buffer ID must be 0
Buffer Offset must be 0
Allocation length must be set to between 0 and 516 (4 bytes of header
and up to 512 bytes of data).
0010b Data Only.– In this mode, the data-in buffer transfer contains data from
the controller’s memory.
Buffer ID. When non-zero, the value specifies a specific track line in
the controller’s cache. If Buffer ID of zero is specified, the buffer
referenced is the same as if a Buffer ID of one were specified.
Buffer Offset. When non-zero, the value of Buffer Offset can range
from 0 to one less than the number of sectors/track multiplied by 512.
This value must be either a zero or a multiple of 512.
Allocation Length Must be between zero and the maximum number
of sectors/track multiplied by 512.
0011b Descriptor Only
When requesting a descriptor (a description of the buffer referenced in
the Buffer ID field), the resulting data is the offset boundary
requirements and the length of each buffer.
Buffer ID When non-zero, the value specifies a specific track line within
the controller’s cache. If Buffer ID of 0 is specified, the buffer referenced
is the same as if a Buffer ID of 1 were specified.
Buffer Offset Must be zero
Allocation Length Must be between 4 (header) and the maximum
number of sectors/track multiplied by 512 + header size.
1010b Read Data from Echo Buffer
In this mode, the drive transfers data to the host from the echo buffer.
The echo buffer will transfer the same data as when the WRITE BUFFER
command with the mode field set to echo buffer was issued.
Buffer ID Ignored.
Buffer Offset Ignored.