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Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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4.0
SCSI commands
This section defines the SCSI command structure and describes a typical SCSI bus procedure involving a
command, status return and message interchange.
The command structure defined herein provides for a contiguous set of logical blocks of data to be trans-
ferred across the interface. The number of logical data blocks to be transferred is defined in the command.
Initiator commands to the disc drive are structured in accordance with the requirements imposed by the disc
drive physical characteristics. These physical characteristics are reported to the initiator in response to an
inquiry command.
A single command may transfer one or more logical blocks of data. The disc drive may disconnect, if allowed
by the initiators, from the SCSI bus to allow activity by other SCSI devices while the disc drive performs
operations within itself.
Upon command completion (which may be executed either successfully or unsuccessfully), the disc drive
returns a status byte to the initiator. Since most error and exception conditions cannot be adequately de-
scribed with a single status byte, one status code that can be sent as the status byte is called Check Condi-
tion. It indicates that additional information is available. The initiator may issue a Request Sense command to
request the return of the additional information as part of the Data In phase of the command.
4.1
Command implementation requirements
The first byte of any SCSI command contains an operation code as defined in this document. Three bits (bits
7-5) of the second byte of each SCSI command specify the logical unit if it is not specified using the Identify
Message (see Paragraph 3.5.3.1). Only logic unit zero is valid for disc drives to which this specification
applies. The last byte of all SCSI commands shall contain a control byte as defined in Paragraph 4.2.6.
4.1.1
Reserved
Reserved bits, bytes, fields, and code values are set aside for future standardization. Their use and interpre-
tation may be specified by future revisions to this specification. A reserved bit, field, or byte shall be set to
zero, or in accordance with a future revision to this specification. A disc drive that receives a reserved code
value shall terminate the command with a Check Condition status and the Sense Key shall be set to Illegal
Request. It shall also be acceptable for the disc drive to interpret the bit, field, byte, or code value in accor-
dance with a future revision to this specification.
4.2
Command descriptor block (CDB)
A request by an initiator to a disc drive is performed by sending a Command Descriptor Block (CDB) to the
disc drive. For several commands, the request is accompanied by a list of parameters sent during the Data
Out phase. See the specific commands for detailed information.
The Command Descriptor Block always has an operation code as the first byte of the command. This is
followed by a logical unit number, command parameters (if any), and a control byte.
For all commands, if there is an invalid parameter in the Command Descriptor Block, the disc drive shall
terminate the command without altering the medium.
The format description for the Command Descriptor Block as supported by the disc drive is shown in Tables
4.2-1, 4.2-2 and 4.2-3.
