M2488 PRODUCT GUIDE TAPE UNIT SCSI COMMANDS
April 1997 CG00000-011503 REV. A 4-71
Table 4-64. READ BUFFER Command Mode
Vendor Unique Mode (001b) and Data Mode (010b):
In these modes, the DATA IN phase contains buffer data. The Buffer ID field identifies the spe-
cific buffer within the target. The supported Buffer IDs for the vendor unique and data modes are
defined in Table 4-65. Data transfer occurs only within the buffer area indicated by the buffer ID.
If an unsupported buffer ID value is selected, the target returns CHECK CONDITION status and
sets the sense key to ILLEGAL REQUEST with an additional sense code of ILLEGAL FIELD IN
CDB.
Data is transferred beginning at the offset within the buffer as specified by the buffer offset. If the
initiator fails to conform to the offset boundary requirements returned in the READ BUFFER
descriptor, CHECK CONDITION status is returned with a sense key set to ILLEGAL REQUEST
with an additional sense code of ILLEGAL FIELD in CDB.
NOTES:
1. The read/write data buffer and NVRAM are wrap-around buffers. Therefore, the entire capacity
specified by the Read Buffer Descriptor is available, regardless of the offset specified.
2. Prior to allowing READ BUFFER command processing to occur for the read/write data buffer,
the controller performs required positioning or synchronization. Buffered write data is written
to tape and buffered read data is discarded.
3. There is only one 512-byte NVRAM area available. This area may be accessed by any initia-
tor. The NVRAM is not partitioned into “per initiator” areas.
4. The only difference between the vendor unique mode and data mode is that a READ BUFFER
in vendor unique mode reads the number of bytes requested and does not perform any CRC
checking. A READ BUFFER in data mode reads the number of bytes requested and an addi-
tional 2 bytes of CRC and then verifies the CRC. The CRC is then stripped away and not sent
to the initiator with the other Read Buffer data.
BYTE 1 MODE IMPLEMENTED
BIT 2 BIT 1 BIT 0
0 0 0 Combined header and data No
0 0 1 Vendor unique Yes
0 1 0 Data (Refer to description below.) Yes
0 1 1 Descriptor (Refer to description below.) Yes
1 0 0 Reserved N/A
1 0 1 Reserved N/A
1 1 0 Reserved N/A
1 1 1 Reserved N/A
Table 4-65. Supported Buffer ID Values for Read Data Mode
BUFFER ID DESCRIPTION CAPACITY
0 Read/Write Data Buffer
Specified in the Buffer Capacity field of the Read
Buffer Descriptor obtained via the Read Buffer
command.
1 Read/Write nonvolatile (NV) RAM
Specified in the Buffer Capacity field of the Read
Buffer Descriptor obtained via the Read Buffer
command (512 bytes).
