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THEORY OF OPERATION
6.8.2 Write Buffer Underrun
To write in the streaming mode, the tape must be in constant motion. For tape motion to be
constant, the flow of data from the host must be sufficient to keep the drive buffers full of data.
If
the drive has written block N and the next buffer of data N + 1 is not ready to be written, an
underrun sequence begins. The drive begins writing block N a second time and continues the
read-after-write check of the first copy of block N.
If
a buffer of data is still not ready when the
drive completes the read check, the drive makes the decision to stop the tape if N was written
successfull y.
Before stopping the tape, the drive finishes the second copy of N and begins writing a gap.
When the gap reaches the read head, the drive stops tape motion and repositions the tape in
preparation for writing new data.
When the drive receives its next full block of data,
it
starts tape motion, reads correct tape
speed, and begins writing new data after the gap. This underrun procedure allows about 3.33
mSec (at 90 ips) of delay for the host to catch up to the drive before tape motion is interrupted.
It
also ensures that the drive will wait for the host if something interrupts data flow.
6.8.3 Read Buffer Underrun
In a normal read operation, the drive reads a block of data from the tape into the buffer.
It is
then transferred via the bus, from the drive buffer to the host. The buffers are allocated so that
one buffer is used for data being read in, a second buffer to transfer data to the host, and the
third in reserve in case the host gets behind.
If
the host fails to empty the drive buffers, a read buffer underrun may occur. An underrun
occurs when the read channel has located the next block of data and none of the buffers are
available.
To prevent the loss of this next block of data, the Viper drive controller stops the tape, reverses
tape direction, then positions the tape in preparation for continuing reading. When the next
buffer becomes available, the drive gets the tape up to speed and begins reading again.
6-23
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