Tandberg Data
Logical Characteristics
3-9
Tandberg SLR Product Line SCSI Functional Specifications
3.3.
SCSI Pointers
The SCSI architecture provides for two sets of three
pointers within each Initiator. The first set of
pointers is known as the current (or active) pointers.
The second set of pointers is known as the saved
pointers.
There are three kind of pointers in a pointer set. There
are pointers pointing to a command, data or status area
in the memory of the Initiator.
The pointers in the current pointer set are used to
represent the state of the interface and point to the
command, data and status byte to be transferred between
the Initiator memory area and the Target for the current
connection. There is only one set of current pointers in
each Initiator. The current pointers are valid for the
Target currently connected to the Initiator.
The pointers in the saved pointer sets represent the
state of each command that the Initiator has active
(whether or not it is currently connected). If the
Initiator for instance has four different commands
active in one or more targets (including the currently
connected command), there will be four complete sets of
saved pointers stored in the Initiator.
The saved command pointer always points to the start of
the Command Descriptor Block for the current command.
At the beginning of each command, the saved data pointer
points to the start of the data area. It remains at this
value until the Target sends a SAVE DATA POINTER message
to the Initiator. In response to this message, the
Initiator stores the value of the current data pointer
into the saved data pointer.
The saved status pointer always points to the start of
the status area for the current command.
