Scsi Pointer - Fujitsu MBA3073 SERIES Technical Manual

(3) Path establishment of I/O operation
After the SELECTION phase, the IDENTIFY, ABORT TASK SET, or TARGET RESET
message must first be sent from the INIT to the TARG. The IDENTIFY message can be followed
by another message such as a SYNCHRONOUS DATA TRANSFER REQUEST message.
If tagged queuing technique is used, the TASK SET message is issued immediately after the
IDENTIFY message. This IDENTIFY message establishes an I/O operation path for the logical
unit specified by the INIT.
After the end of RESELECTION phase, the TARG shall first send the IDENTIFY message to the
INIT. This IDENTIFY message establishes an I/O operation path for the logical unit specified by
the TARG. If the tagged queuing is used, the SIMPLE message is issued after the IDENTIFY
message.
If the INIT has enabled the disconnect processing and if it has established an I/O operation path
for the specific logical unit (by issuing the IDENTIFY message), the INIT shall set the current
pointer value to the same value (the initial value) of the Saved pointer value of the logical unit.
During reconnection processing (that is, when the IDENTIFY message is issued after the
RESELECTION phase), the pointer is restored implicitly (and the Saved pointer value is set to the
current pointer value).
2.2

SCSI Pointer

The SCSI pointer feature is required by the INIT to control the command execution on the SCSI
bus. It allows multiple TARGs and logical units to process multiple commands concurrently, and
allows the TARG to retry processing in bus phases.
(1) Type of pointers
The following three types of SCSI pointers have been defined:
•
Command pointer: controls and manages the command (CDB) transfer.
•
Data pointer: controls and manages data transfer.
•
Status pointer: controls and manages the status byte transfer.
All INITs must have these three types of pointers listed above. These pointers indicate INIT
memory addresses for status byte storage, data transfer, and command (CDB) fetch when viewed
from the SCSI device functioning as a TARG.
The INIT needs to have one pair of current pointers and several pairs of saved pointers. The
current pointers are used for the command which is being executed by the TARG currently
associated with the INIT. A current pointer value is updated every time one-byte information is
transferred in the INFORMATION TRANSFER phase. On the other hand, there is one pair of
saved pointers for every command issued by the INIT (during its execution on the SCSI bus or
disconnection). The values of the current and saved pointers are identical (initial value) when the
command is issued.
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