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1.3.2
Command Link
The command link function is a function which causes the TARG to execute continuous
processing of multiple commands. Some examples of command link processing are shown
below.
1) The INIT sets the command’s initial values in the command pointer, data pointer and
status pointer.
2) Acquiring of the right to use the SCSI bus, selection of the TARG and specification of the
LUN by the IDENTIFY message are the same as in the case of single commands.
3) The TARG receives commands from the INIT in the COMMAND phase, but “1” is
specified in the Link bit of the CDB’s control byte.
4) The TARG analyzes the command and executes the requested processing.
5) If processing of the command is completed normally, the TARG notifies the INIT of the
INTERMEDIATE status in the STATUS phase. At this time, the command link function
becomes effective.
6) The TARG informs the INIT of the LINKED COMMAND COMPLETE or the LINKED
COMMAND COMPLETE WITH FLAG message, depending on the value of the Flag bit
in the CDB’s control byte. When the INIT has received the LINKED COMMAND
COMPLETE (WITH FLAG) message, the command, data and status pointers are updated
to the initial values for the next command in the link.
7) The TARG enters the COMMAND phase immediately after the MESSAGE IN phase and
receives the command it is to execute next. After that, it performs either single command
(Link bit = “0”) processing or command link (Link bit = “1”) processing.
The command link continues until a command with “0” specified in the Link bit of its CDB is
issued or until a command terminates abnormally.
The command link function is made effective only in the case that commands with link
specifications are completed normally. If a command with a link specification is completed in
an error state or in an exception state, the command link function is invalidated. Table 1.1
shows the response of the IDD when commands with a Link specification are terminated.