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SCSI architecture provides a set of three pointers (called saved pointers) for each
I/O process. The three pointers are: Command, Status, and Data. When an I/O
process becomes active, the three saved pointers are copied to the initiator as
current pointers. There is only one set of current pointers in the initiator at one
time. The current pointers point to the next command, data, or status byte to be
transferred between the initiator’s memory and the drive. The saved and current
pointers reside in the initiator.
The saved command pointer always points to the start of the Command Descriptor
Block for the I/O process. The saved status pointer always points to the start of the
status area of the I/O process. The saved data pointer always points to the start of the
data area until the drive sends a SAVE DATA POINTER message for the I/O
process back to the initiator.
In response to the SAVE DATA POINTER message, the initiator stores the value
of the current data pointer into the saved data pointer for that I/O process. The
drive can restore the current pointer from the saved pointer value for the active I/O
process by sending a RESTORE POINTERS message to the initiator. The initiator
then copies the set of saved pointers into the set of current pointers. Whenever a
drive disconnects from the SCSI Bus, only the set of saved pointers is retained in
the initiator. The set of current pointers is restored from the set of saved pointers
when the I/O process is reconnected.