DATA TRANSFER COMMANDS
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.
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 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.
Table 20 - Sector Sizes
N
SECTOR SIZE
00
01
02
03
..
128 bytes
256 bytes
512 bytes
1024 bytes
...
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.
If the FDC detects a pulse on the nINDEX pin twice
without finding the specified sector (meaning that the
diskette's index hole passes through index detect logic in
the drive twice), the FDC sets the IC code in Status
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