Specifications

5.4.10 Read Sectors command
This command (command codes 20
H
and 21
H
) reads from 1 to 256
sectors as specified in the Sector Count register (a sector count of 0
requests 256 sectors), beginning at the specified sector.
If the drive is not already on the desired track, an implied seek is
performed. Once on the desired track, the drive searches for the re-
quested sector. If the requested sector cannot be found, an ID Not Found
or Address Mark Not Found error is posted.
DRQ is always set prior to data transfer regardless of the presence or
absence of an error condition. At command completion, the Command
Block registers contain the cylinder, head and sector number of the last
sector read.
If a data error occurs, the read terminates at the sector where the error
occurred. For example, if a READ command of twenty sectors encoun-
ters an uncorrectable data error in sector eight, then after reading sector
eight, the flawed data would be stored in the sector buffer and the
command would terminate. The Command Block registers would contain
the cylinder, head and sector number of the sector where the error
occurred. If the no-retry command code (21
H
) is issued, some or all of
the error recovery procedures normally used with this command may be
skipped.
5.4.11 Read Verify Sectors command
This command (command codes 40
H
and 41
H
) is identical to the Read
Sectors command, except that DRQ is never set, and no data is trans-
ferred to the host.
When the requested sectors have been verified, the drive clears BSY
and generates an interrupt. Upon command completion, the Command
Block registers contain the cylinder, head, and sector number of the last
sector verified.
If an error occurs, the verify ends at the sector where the error occurs.
The Command Block registers contain the cylinder, head and sector
number of the sector where the error occurred. The Sector Count register
contains the number of sectors not yet verified. If the no-retry command
code (41
H
) is issued, some or all of the error recovery procedures
normally used with this command may be skipped.
ATA Interface Reference Manual, Rev. C 49