
54
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 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.
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 23 describes the effect
of the SK bit on the Read Data command
execution and results. Except where noted in
Table 23, the C or R value of the sector address
is automatically incremented (see Table 25).
Table 22 - Effects of MT and N Bits
MT
N
MAXIMUM TRANSFER
CAPACITY
FINAL SECTOR READ
FROM DISK
0
1
0
1
0
1
2
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 23 - Skip Bit vs Read Data Command
SK BIT
VALUE
DATA ADDRESS
MARK TYPE
ENCOUNTERED
RESULTS
SECTOR
READ?
CM BIT OF
ST2 SET?
DESCRIPTION
OF RESULTS
0
1
Normal Data
Deleted Data
Normal Data
Deleted Data
Yes
No
Yes
No
Yes
Normal
termination.
Address not
incremented.
Next sector not
searched for.
Normal
termination.
Normal
termination.
Sector not read
("skipped").