
61
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 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 20
describes the effect of the SK bit on the Read Data command execution and results. Except where
noted in Table 20, the C or R value of the sector address is automatically incremented (see Table 22).
Table 18 – Effects of MT and N Bits
MAXIMUM TRANSFER
CAPACITY
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
MT
0
1
0
1
0
1
N
1
1
2
2
3
3
FINAL SECTOR READ
FROM DISK
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 19 – Skip Bit vs. Read Data Command
DATA ADDRESS
MARK TYPE
ENCOUNTERED
SECTOR
READ
Normal Data
Yes
SK BIT
VALUE
RESULTS
CM BIT OF
ST2 SET
No
DESCRIPTION
OF RESULTS
Normal
termination.
Address not
incremented.
Next sector not
searched for.
Normal
termination.
Normal
termination.
Sector not read
("skipped").
0
0
Deleted Data
Yes
Yes
1
Normal Data
Yes
No
1
Deleted Data
No
Yes
Read Deleted Data
This command is the same as the Read Data command, only it operates on sectors that contain a
Deleted Data Address Mark at the beginning of a Data Field.