Section 5 SCSI Programming Guidelines
5.11.7.2 WRITE Command Operation
WRITE
commands
on
the ProDrive are buffered using a 64K-byte buffer. Data
to
be stored on the disk.
is first stored in the buffer. When the head is correctly positioned
on
the track
over
the appropriate data
area, data will be written from the buffer to the disk.
The
two operations involved in writing, transferring data from the SCSI bus to the buffer and transferring
data from the buffer onto the disk, are essentially independent processes.
The
first operation. transferring
data
from the SCSI bus into the buffer, will continue as long as the transfer length indicates that there is
more data
to
be
transferred across the SCSI
bus
from the initiator and there is room in the buffer.
If
the
buffer fills completely, the transfer
of
data across the SCSI bus will temporarily halt.
The
second
operation, transferring data from the buffer
onto
the disk, will occur as long as the buffer is not empty and
the head is correctly positioned
over
the appropriate data area.
The
only interaction between these
two
operations involves the buffer level.
The
"buffer-te-disk operation will stop
if
the SCSI bus-to-buffer
transfer process cannot keep up
and
the buffer empties. In
tum,
the SCSI-to-buffer transfer will
temporarily halt
if
the buffer-to-disk transfer process falls behind and the buffer fills
completely.
The
disconnect/reconnect strategy is tied to the dual processes
of
filling and emptying the buffer. The
ProDrive will
not
disconnect unless the initiator sends
an
IDENTIFY message with the "I Support
Disconnect"
bit
(bit 6) set.
If
disconnect is supported by the initiator, the drive will disconnect whenever it
can
no longer transfer data from the SCSI bus to the buffer because the buffer is full
(or
the transfer count is
exhausted).
The
point at which reconnection occurs is controlled by the Buffer Empty Ratio (MODE
SELECT
Page 2 byte 3). This ratio determines
how
low the buffer level must get
via
data transfer
to
disk
before reconnection is attempted. After a successful reconnection, the
SCSI bus-to-buffer transfer operation
will resume.
5.11.8
SEEK COMMAND: OPCODE = OB(hex)
The
SEEK
command (Table 5-20) requests that the drive seek to the specified logical block address.
TABLE
5·20
SEEK
Command
Bit
7
6
5
4
3
2
1
o
Byte
0
OPCODE=OBH
1
LUN=O
I (MSB) LOGICAL BLOCK ADDRESS
2-3 LOGICAL BLOCK ADDRESS
MB)
4 RESERVED
=0
5
VU=O
I
RESERVED
=0
I
F
I
L
LOGICAL BLOCK ADDRESS (Byte 1, Bits
S-O;
Bits 2·3) - Specifies
the
logical
block
to which the actuator will
be
repositioned after the
SEEK
operation is complete.
5·38
SCSI
Programming
Guidelines
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