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.
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.
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.
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 is reflected in the results
status normally returned for a Read/Write Data
command. Status Register 0 (ST0) would contain
the error code and C would contain the cylinder on
which the seek failed.
N determines the number of bytes per sector (see
Table 21 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.
Read Data
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
TABLE 22 - SECTOR SIZES
N
00
01
02
03
..
SECTOR SIZE
128 bytes
256 bytes
512 bytes
1024 bytes
...
07
16 Kbytes
at Sector 1, Side 0 and completing the last sector
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).
of the same track at Side 1.
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 20.
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 the completion of the Read Data command, the
head is not unloaded until after the Head Unload
Time Interval (specified in the Specify command)
53
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