Interface
(2)READ SECTOR(S) (X '20' or X '21')
This command reads data of sectors specified in the Sector Count field from the address specified in the Device/Head, Cylinder High, Cylinder Low and Sector Number fields. Number of sectors can be specified from 1 to 256 sectors. To specify 256 sectors reading, '00' is specified. For the protocols related to data transfer, see Subsection 5.4.1.
If the head is not on the track specified by the host, the device performs an implied seek. After the head reaches to the specified track, the device reads the target sector.
If an error occurs, retry reads are attempted to read the target sector before reporting an error, irrespective of the R bit setting.
If an error does not occur, PIO Setup is always transferred prior to the data transfer.
Upon the completion of the command execution, shadow block registers contain the cylinder, head, and sector addresses (in the CHS mode) or logical block address (in the LBA mode) of the last sector read.
If an unrecoverable disk read error occurs in a sector, the read operation is terminated at the sector where the error occurred. Shadow block registers contain the cylinder, the head, and the sector addresses of the sector (in the CHS mode) or the logical block address (in the LBA mode) where the error occurred, and remaining number of sectors of which data was not transferred (including sector when the error occurred).
•Error reporting conditions
(1)A specified address exceeds the range where read operations are allowed (ST = 51h, ER = 10h).
(2)The range where read operations are allowed will be exceeded by an address during a read operation (ST = 51h, ER = 10h).
(3)An uncorrectable disk read error occurred (ST = 51h, ER = 40h).
(4)The sync byte indicating the beginning of a sector was not found (ST = 51h, ER = 01h).
(5)A communication error occurred (ST = 51h, ER = 0Ch).
(6)An error other than the above errors occurred (ST = 51h, ER = 04h).
