Seagate Ultra 160 Command implementation requirements, Command Descriptor Block CDB, Reserved

7.0SCSI 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 transferred across the interface. The number of logical data blocks to be transferred is defined in the command. Initiator commands to the drive are structured in accordance with the requirements imposed by the drive physical char- acteristics. 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 drive may disconnect, if allowed by the initiators, from the SCSI bus to allow activity by other SCSI devices while the drive performs operations within itself.

Upon command completion (which may be executed either successfully or unsuccessfully), the drive returns a status byte to the initiator. Since most error and exception conditions cannot be adequately described with a single status byte, one status code that can be sent as the status byte is called Check Condition. 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.

Future implementations of the SCSI protocol will contain an autosense feature (see Section 7.6.4.2) allowing the application client to request the automatic return of sense data. Fibre Channel protocol already has this feature in its packetized command structure.

7.1Command 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 have historically been used to specify the logical unit if it is not specified using the Identify Message (see Section 4). These three bits are now shown as “Reserved” in the new Command data block format. The last byte of most SCSI commands contains a control field. The excep- tion to this is the new variable length CDB which has the control field in the second byte, rather than the last.

7.1.1Reserved

Reserved bits, bytes, fields, and code values are set aside for future standardization. Their use and interpreta- tion 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 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 drive to interpret the bit, field, byte, or code value in accordance with a future revision to this specification.

7.2Command Descriptor Block (CDB)

A request by a SCSI initiator port to a drive is performed by sending a Command Descriptor Block (CDB) to the drive. For several commands, the request is accompanied by a list of parameters sent during the DATA OUT phase. The field uses shown in tables 57, 58, 59, 60, and most of those in 61 are used consistently by most commands. See the specific commands for detailed information.

Parallel SCSI Interface Product Manual, Rev. A 131