Seagate Fibre Channel Interface - Manual (Page 146)

132 Fibre Channel Interface Manual, Rev. D
Logical Block Address
See Section 11.2.1.2.
Operation Code
See Section 12.0.
RelAdr (Relative Address)
See Section 11.2.1.4.
Transfer Length
See Section 11.2.1.5
11.2.1.2 Logical block address
The logical block address in the Command Descriptor Block begins with block zero and is continuous up to the
last logical block on drive.
Group 0 command descriptor block contains 21-bit logical block addresses. Groups 1 and 2 command descrip-
tor blocks contain 32-bit logical block addresses.
The logical block concept implies that the initiator and target have previously established the number of data
bytes per logical block. This may be established through the use of the Read Capacity command or the Mode
Sense command or by prior arrangement.
The maximum logical block address for the disc drive which is accessible by the initiator is defined in Read
Capacity Command data in Section 12.26.
11.2.1.3 Operation code
Operation codes are defined in Section 12.0.
11.2.1.4 Relative address bit
Relative addressing is a technique of accessing logical blocks relative to the logical blocks accessed in a previ-
ous linked command. Seagate fibre channel drives do not support relative addressing.
11.2.1.5 Transfer length
The Transfer Length field specifies the amount of data to be transferred, usually the number of blocks. For sev-
eral commands, the Transfer Length indicates the requested number of bytes to be sent as defined in the com-
mand description. For these commands, the Transfer Length field may be identified by a different name. See
the following descriptions and the individual command descriptions for further information.
Commands that use one byte for the Transfer Length field allow up to 256 blocks of data to be transferred by
one command. A Transfer Length field value of 1 to 255 indicates the number of blocks that are transferred. A
value of zero indicates 256 blocks.
Commands that use two bytes for the Transfer Length field allow up to 65,535 blocks of data to be transferred
by one command. In this case, a Transfer Length of zero indicates that no data transfer takes place. A value of
1 to 65,535 indicates the number of blocks that are transferred.
For several commands, more than two bytes are allocated for the Transfer Length field. Refer to the specific
command description for further information.
The Transfer Length field of the commands that are used to send a list of parameters to a disc drive is called
the Parameter List Length field. The Parameter List Length field specifies the number of bytes sent during the
FCP DATA sequences for the command.
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