Atari 400 Technical Reference Manual page 83

Each of the DCB bytes will now be described, and the
system-equate file name for each will be given.
SERIAL BUS ID -- DDEVIC [0300J
The Diskette Handler sets up this byte to contain the Serial Bus ID
for the drive to be accessed. It is not user-alterable.
DEVICE NUMBER -- DUNIT [0301J
You set up this byte to contain the disk drive number to be
accessed (1 - 4).
COMMAND BYTE -- DCOMND [0302J
You set up this byte to contain the disk device command to
be performed.
STATUS BYTE -- DSTATS [0303]
This byte contains the status of the command upon return to the
caller. See Appendix C for a list of the possible status codes.
BUFFER ADDRESS -- DBUFLO [0304] and DBUFHI [0305]
This 2-byte pointer contains the address of the source or
destination of the diskette sector data. You need not supply
an address for the disk status command. The Disk Handler will
obtain the status and insert the address of the status buffer
into this field.
DISK TIMEOUT VALUE -- DTIMLO [0306]
The Handler supplies this timeout value (in whole seconds) for
use by SIO.
BYTE COUNT -- DBYTLO [0308] and DBYTHI [0309]
This 2-byte counter indicates the number of bytes transferred to
or from the disk as a result of the most recent command, and is
set up by the Handler.
SECTOR NUMBER -- DAUX1 [030AJ and DAUX2 [030B]
This 2-byte number specifies the diskette sector number (1 - 720)
to read or write. DAUX1 contains the least significant byte, and
OPERATING SYSTEM C016555 -- Section 5