Viper 60s Theory Of Operation And Maintenance Manual page 56

INTERFACE
4.23 SCSI Memory Address Pointers
SCSI provides for two sets of three pOinters within each Initiator. When a physical path is
established with a host, and this path can accommodate disconnection and reconnection,
the host must ensure that its Current Pointers for the path are equal to the Saved Pointers
in the Viper drive. An implied Restore Pointers Operation occurs in the host as a result of
a connect or reconnect.
4.23.1 Current Data Pointers
Also known as Active Pointers, these pointers are used to represent the state of the
interface and pOint to the next Command, Status or Data byte to be transferred between
the memory of the Initiator and the Target. There is only one set of Current Pointers in
each Initiator. The Current Pointers are used by the Target currently connected to the
Initiator.
4.23.2 Saved Data Pointers
There is one set of these pointers for each currently active device whether or not it is
currently connected. The set includes Command, Status and Data Pointers that point to
the Command Descriptor Block, Status Area and Data Area, respectively, for that device.
The Saved Pointer continues to pOint to the start of the Data Area until the Target
reconnects to the Initiator. In response to the implied Save Data Pointer message, the
Initiator replaces the Current (active) Data Pointer with the value of the Saved Data
Pointer.
4.24 Early Warning Function
Early Warning on the Viper drive is a logical warning given when one megabyte of
storage space remains on the tape. The position is calculated by the drive based on the
amount of data the drive was able to store on a track. When this physical position is
reached on a tape, the following occurs.
1. Data transfers from the host are terminated at the next disconnect boundary.
2. All data remaining in the drive buffer is written to the tape.
3. The command completes with 40h Sense data meaning EOM and no Sense Key.
4. Subsequent Write commands will write data and complete with check condition
with EOM Status and No Sense Key until the physical tape holes are encountered.
See Write Command for a description.
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