1. SCSI Reference Guide
  2. SCSI Commands
  3. Command Descriptor Block (CDB) Structure

Command Descriptor Block (CDB) Structure

  • The first byte contains the operation code — a Group Code that provides eight groups of commands and a Command Code that provides 32 command codes for each group.

  • The second byte starts the command parameters.

  • The last byte is the control byte (see Control Byte Structure).

For some commands, a list of parameters accompanies the request during data out. For all commands, if there is an invalid parameter in the CDB, then the library terminates the command without altering the medium.

Figure 2-1 6-Byte Command Structure

Description of Figure 2-1 follows
Description of "Figure 2-1 6-Byte Command Structure"

Figure 2-2 10-Byte Command Structure

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Description of "Figure 2-2 10-Byte Command Structure"

Figure 2-3 12-Byte Command Structure

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Description of "Figure 2-3 12-Byte Command Structure"

Control Byte Structure

The control byte is the last byte of every CDB.

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Description of the illustration slk_068.png
Vendor Specific

Provides information about the device.

NACA (Normal auto contingent allegiance)

Controls the rules for handling an auto contingent condition caused by a command. When NACA is 0, the command will return a check condition if a contingent allegiance condition occurs.

Flag (not supported)

Causes an interrupt in the initiator allowing a device to respond with intermediate status. This bit is should be 0.

Link (not supported)

Allows devices that support command linking to continue the I/O process. This bit should be 0.