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An initiator communicates with the drive by sending a 6- or 10-byte Command
Descriptor Block that contains the parameters for the specific command. The SCSI
command’s operation code is always the first byte in the Command Descriptor
Block and a control field is the last byte. For some commands, the Command
Descriptor Block is accompanied by a list of parameters sent during the DATA
OUT phase. Figure 5–1 shows the format of a typical 6-byte Command Descriptor
Block. Table 5–2 contains a description of the Command Descriptor Block fields.
Bit
Byte
76543210
0 Operation Code
1 Logical Unit Number
(LUN)
(MSB)
Logical Block Address
(LBA)
2 - 3 Logical Block Address
(LBA) (LSB)
4 Transfer Length, Parameter List Length, or Allocation Length
5 Control
NOTE: Unless otherwise specified, all reserved bits indicated in the
commands are 0. The Control byte is described on Page 5-6.
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