Fujitsu MAA3182SC Oem Manual page 59

INIT
Write→
←Read
Figure 2.1
The basic functions and operations of the data buffer in a read operation and a write operation
are shown below.
(1) Read Operation
After data read from the disk media are temporarily stored in the data buffer, they are
transferred to the SCSI bus with the timing specified in the MODE SELECT parameter. The
basic read operation procedure is as shown below.
1) When the IDD receives a command, normally, it disconnects from the SCSI bus through
disconnect processing and locates the data block specified in the command where it is
stored on the disk media.
2) When the target data block has been located, the IDD reads the data form the disk media to
the data buffer. At this time, if a recoverable error is detected, the IDD performs error
recovery processing according to the specification in the MODE SELECT parameter or
corrects the data in the data buffer.
3) When the IDD has read the amount of data specified by the buffer full ratio in the MODE
SELECT parameter (see item 2.1.2) to the data buffer, it performs reconnection processing
with the SCSI bus and begins transferring data to the INIT (SCSI bus) from the data buffer
while continuing to read subsequent data blocks to the data buffer.
4) If the data transfer capacity of the INIT (SCSI bus) is high and the data buffer becomes
empty before completing transfer of the number of data blocks specified by the command,
the IDD performs disconnect processing at that point. Thereafter, reconnection, data
transfer and disconnection (the operations in 3) and 4) above) are repeated until all the data
blocks specified in the command have been transferred.
5) If the number of blocks specified in the command to be transferred is larger than the
capacity of a cache segment and if the data transfer rate of the INIT is lower than the data
transfer rate of the disk drive, the empty space in the data buffer disappears from step 3)
and the IDD may soon not be able to read data from the disk media (data overrun). In this
case, the IDD relocates the block that caused the data overrun after waiting one disk
revolution, and continues the reading process.
6) When transfer of all the specified data is completed, the IDD reports the status and
terminates the command.
2 - 2
Data Buffer (512 KB FIFO)
Valid Data
(If data buffer 0 is selected)
Data Buffer Configuration (In the case of 4 cache segments)
C141-E039-01EN
Buffer 3
Buffer 2
Buffer 1
Buffer 0
Write→
←Read
Disk Media