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.
After reading the ID and Data Fields in each
sector, the FDC checks the CRC bytes. If a
CRC error occurs in the ID or data field, the
FDC sets the IC code in Status Register 0 to
"01" indicating abnormal termination, sets the
DE bit flag in Status Register 1 to "1", sets the
DD bit in Status Register 2 to "1" if CRC is
incorrect in the ID field, and terminates the Read
Data Command. Table 24 describes the effect
of the SK bit on the Read Data command
execution and results. Except where noted in
Table 24, the C or R value of the sector address
is automatically incremented (see Table 26).
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 Register 0 to
"01" indicating abnormal termination, sets the
ND bit in Status Register 1 to "1" indicating a
sector not found, and terminates the Read Data
Command.
Table 23 - Effects of MT and N Bits
MT
N
MAXIMUM TRANSFER
CAPACITY
FINAL SECTOR READ
FROM DISK
0
1
0
1
0
1
1
1
2
2
3
3
256 x 26 = 6,656
256 x 52 = 13,312
512 x 15 = 7,680
512 x 30 = 15,360
1024 x 8 = 8,192
1024 x 16 = 16,384
26 at side 0 or 1
26 at side 1
15 at side 0 or 1
15 at side 1
8 at side 0 or 1
16 at side 1
Table 24 - Skip Bit vs Read Data Command
DATA ADDRESS
MARK TYPE
ENCOUNTERED
SK BIT
VALUE
RESULTS
SECTOR CM BIT OF DESCRIPTION OF
READ?
ST2 SET?
RESULTS
0
0
1
Normal Data
Deleted Data
Normal Data
Yes
No
Normal termination.
Address not incremented.
Next sector not searched
for.
Yes
Yes
Yes
No
Normal termination.
1
Deleted Data
No
Yes
Normal termination. Sector
not read ("skipped").
56
SMSC [ SMSC CORPORATION ]
