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INTERFACE COMMANDS
7 – 2
Read Commands
Read Sector(s)
Reads from 1 to 256 sectors, as specified in the Command Block, beginning at the specified sector. (A sector
count of 0 requests 256 sectors.) Immediately after the Command register is written, the drive sets the BSY
bit and begins execution of the command. If the drive is not already on the desired track, an implied seek is
performed.
Once at the desired track, the drive searches for the data address mark of the requested sector. The data
address mark must be recognized within a specified number of bytes, or the Data Address Mark Not Found
error will be reported. Assuming the data address mark is found:
1. The data field is read into the sector buffer.
2. Error bits are set (if an error was encountered).
3. The DRQ bit is set.
4. An interrupt is generated.
The DRQ bit is always set, regardless of the presence or absence of an error condition after the sector.
Upon command completion, the Command Block registers contain the numbers of the cylinder, head and
sector of the last sector read. Back-to-back sector read commands set DRQ and generate an interrupt when
the sector buffer is filled at the completion of each sector. The drive is then ready for the data to be read by
the host. DRQ is reset and BSY is set immediately when the host empties the sector buffer.
If an error occurs during Read Sector commands, the read terminates at the sector where the error occurred.
The host may then read the Command Block to determine the nature of that error, and the sector where it
happened. If the error type is a correctable or an non-correctable data error, the flawed data is loaded into
the sector buffer.
A Read Long command sets the Long bit in the command code and returns the data and the ECC bytes in
the data field of the specified sector. During a Read Long, the drive does not check the ECC bytes to
determine if there has been a data error. The Read Long command is limited to single sector requests.
Read Verify Sector(s)
Identical to the Read Sector(s) command, except that:
1. DRQ is never set,
2. No data is transferred back to the host and
3. The long bit is not valid.