N determines the number of bytes per sector
(see Table 22 below). If N is set to zero, the
sector size is set to 128. The DTL value
determines the number of bytes to be
transferred. If DTL is less than 128, the FDC
transfers the specified number of bytes to the
host. For reads, it continues to read the entire
128-byte sector and checks for CRC errors. For
writes, it completes the 128-byte sector by filling
in zeros. If N is not set to 00 Hex, DTL should
be set to FF Hex and has no impact on the
number of bytes transferred.
DATA TRANSFER COMMANDS
All of the Read Data, Write Data and Verify type
commands use the same parameter bytes and
return the same results information, the only
difference being the coding of bits 0-4 in the first
byte.
An implied seek will be executed if the feature
was enabled by the Configure command. This
seek is completely transparent to the user. The
Drive Busy bit for the drive will go active in the
Main Status Register during the seek portion of
the command. If the seek portion fails, it will be
reflected in the results status normally returned
Table 20 - Sector Sizes
N
SECTOR SIZE
for
a
Read/Write Data command. Status
00
01
02
03
..
128 bytes
256 bytes
512 bytes
1024 bytes
...
Register 0 (ST0) would contain the error code
and C would contain the cylinder on which the
seek failed.
Read Data
07
16 Kbytes
A set of nine (9) bytes is required to place the
FDC in the Read Data Mode. After the Read
Data command has been issued, the FDC loads
the head (if it is in the unloaded state), waits the
specified head settling time (defined in the
Specify command), and begins reading ID
Address Marks and ID fields. When the sector
address read off the diskette matches with the
sector address specified in the command, the
FDC reads the sector's data field and transfers
the data to the FIFO.
The amount of data which can be handled with
a single command to the FDC depends upon
MT (multi-track) and N (number of bytes/sector).
The Multi-Track function (MT) allows the FDC to
read data from both sides of the diskette. For a
particular cylinder, data will be transferred
starting at Sector 1, Side 0 and completing the
last sector of the same track at Side 1.
After completion of the read operation from the
current sector, the sector address is
incremented by one and the data from the next
logical sector is read and output via the FIFO.
This continuous read function is called "Multi-
Sector Read Operation". Upon receipt of TC, or
an implied TC (FIFO overrun/underrun), the
FDC stops sending data but will continue to
read data from the current sector, check the
CRC bytes, and at the end of the sector,
terminate the Read Data Command.
If the host terminates a read or write operation
in the FDC, the ID information in the result
phase is dependent upon the state of the MT bit
and EOT byte. Refer to Table 23.
At the completion of the Read Data command,
the head is not unloaded until after the Head
Unload Time Interval (specified in the Specify
command) has elapsed. If the host issues
another command before the head unloads,
then the head settling time may be saved
between subsequent reads.
57
SMSC [ SMSC CORPORATION ]
