Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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4.2.3
Logical Block Address
The logical block address in the command descriptor block shall begin with block zero and be continuous up
to the last logical block on disc drive.
Group 0 command descriptor block contains 21 bit logical block addresses. Groups 1 & 2 command descrip-
tor blocks contain 32 bit logical block addresses.
The logical block concept implies that the initiator and target shall 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 5.2.2.1.
4.2.4
Relative address bit
Relative addressing is a technique useful in accessing structured data in a uniform manner. Relative ad-
dressing is only allowed when commands are linked. Details are given with those commands that use this
feature.
4.2.5
Transfer length
The Transfer Length specifies the amount of data to be transferred, usually the number of blocks. For several
commands the transfer length indicates the requested number of bytes to be sent as defined in the command
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 Transfer Length allow up to 256 blocks of data to be transferred by one
command. A Transfer Length value of 1 to 255 indicates the number of blocks that shall be transferred. A
value of zero indicates 256 blocks.
Commands that use two bytes for Transfer Length 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 shall take place. A value of
1 to 65,535 indicates the number of blocks that shall be transferred.
For several commands more than two bytes are allocated for Transfer Length. Refer to the specific command
description for further information.
The Transfer Length of the commands that are used to send a list of parameters to a disc drive is called the
Parameter List Length. The Parameter List Length specifies the number of bytes sent during the Data Out
phase.
The Transfer Length of the commands used to return sense data (e.g. Request Sense, Inquiry, Mode Sense,
etc) to an initiator is called the Allocation Length. The Allocation Length specifies the number of bytes that the
initiator has allocated for returned data. The disc drive shall terminate the Data In phase when Allocation
Length bytes have been transferred or when all available data have been transferred to the initiator, which-
ever is less.
