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No mechanical or absolute position information is used in the Q250j
Q280, as the actuator finds its position by reading information from
the servo wedges.
At power-up, the position is determined by seeking
toward the middle of the disk, then braking and reading the wedges
while the radial velocity is zero.
Once the actuator position is known, the drive is recalibrated: the
actuator seeks to the outer, middle, and inner tracks, and at each
position, the peak amplitudes of the servo bursts are read.
From this
information, the adaptive gain parameters are calculated and written
into buffer RAM.
The W-list, or working list of defective sectors, is
then read from reserved tracks into buffer RAM.
A duplicate copy of
the list is stored on each surface so that it is always available,
even if one or more copies is damaged.
Finally, the drive seeks to
cylinder 0, and is ready.
2.5
Read Command Sequence
This section explains the special features of the Read Command
Sequence, Figure 2-16.
For additional information, see the Q200
Series Programmers Manual.
The READ command contains the starting logical block address and the
transfer length in blocks.
Before reading each sector, the Q250jQ280
checks the defect list and seeks to the correct position.
After each
sector is read into buffer, the data is checked for correctness; the
action taken is that specified by the Error Recovery Parameters of the
MODE SELECT command.
2.6
write Command Sequence
This section explains the special features of the write Command
Sequence, Figure 2-17.
For additional information, see the Q200
Series Programmers Manual.
The WRITE command contains the starting logical block address and the
transfer length in blocks.
Before writing each sector, the Q250jQ280
checks the defect list and seeks to the correct position.
Writing to disk doesn't start until a complete block is available in
the buffer.
Then, the Q250jQ280 writes to the disk from buffer, and
transfers data from the SCSI bus to buffer almost simultaneously
(actually interleaved, a byte at a time).
When the full transfer
length of data has been transferred from the SCSI bus to the buffer,
and from the buffer to the disk, the transfer is complete and the
Q250jQ280 sends Status and Message to the host.
2-33
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