Fujitsu 80, Scsi Logical Interface Specifications

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2.1 Data Buffer
C141-C010 2-3
6) When transfer of all the specified data is completed, the IDD reports the status and terminates the
command.
Note:
In order to avoid frequent repetition of the disconnect/reconnect processing after data transfer
starts on the SCSI bus, and avoid the occurrence of data overruns, it is necessary to set the
buffer full ratio (see Section 2.1.2) of the MODE SELECT parameter so the difference in the
data transfer rates between the INIT (SCSI bus) and the disk drive is maintained in a good
balance.
Figure 2.2 shows an example of the data buffer operating state during a read operation. Details are
explained in Section 2.1.2.
Data Storage Completion Wait
SCSI Bus
Sector
Disk
Amount of Data
in the Buffer
Buffer Full Ratio
DATA IN
Reconnection
Figure 2.2 Example of data buffer operation during read
(2) Write
operation
After data transferred from the INIT are stored temporarily in the data buffer, they are written to the disk
media. The basic write operation procedure is as shown below.
1) When the IDD receives a command and disconnects, it immediately reconnects and enters the DATA
OUT phase and stores data transferred from the INIT to the data buffer (data pre-fetch). At this time,
the IDD locates the position of the data block on the disk media specified by the command in
parallel.
When storing of all the data specified in the command in the data buffer is completed, or if the empty space
in the data buffer is all filled during data storage due to the volume of the specified data, the IDD performs
disconnect processing and disconnects from the SCSI bus.
2) When locating of the target data block on the disk media is completed, the IDD writes the data in the
data buffer to the disk media. If the target data block location operation is completed during the data
pre-fetch processing in 1), writing of data to the disk media is performed in parallel with the data
pre-fetch to the data buffer.