Quantum DLT 8000 Manual descargar pdf (Pagina 147)

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SCSI Commands

Quantum DLT 8000 Tape System 5-3

5.1.1 SCSI Pointers

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.

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