Specifications

3.2 Interface handshaking
The main handshaking signals between the drive and the host are the
busy bit (BSY) and the data request bit (DRQ) (in the status register) and
the interrupt (INTRQ) signal. They can be set in one of the following ways:
• Any reset will cause BSY to be set.
• Writing a command to the command register will also set BSY.
The BSY bit is used to indicate that the controller is busy and should not
be accessed.
The DRQ bit is used to control the data transfer to and from the controller.
The host can read/write the data register only when the DRQ bit is set to 1.
The INTRQ signal is generated by the drive to interrupt the host.
For example, during a Read Sector command, the drive generates an
INTRQ to the host whenever a sector is ready for the host to read. No
INTRQ is generated immediately after completion of a Read command.
The number of interrupts equals the number of sectors read.
During a Write Sector command, the drive generates an INTRQ when-
ever the drive requests data from the host (except for the first sector).
The drive also generates an interrupt immediately after completion of a
Write command. The number of interrupts equals the number of sectors
written.
All commands that do not include read- or write-data transfers generate
a single interrupt when the command completes. Resets do not generate
an interrupt.
INTRQ is cleared when the host reads the status register. The host can
read the Alternate Status register without clearing the INTRQ.
12 ATA Interface Reference Manual, Rev. C