Atari 400 Technical Reference Manual page 144

Neither Cartridge A nor B is inserted.
Cartridge A is inserted and has bit 0 of its option byte
[BFFDl
=
1.
Cartridge B is inserted and has bit 0 of its option byte
[9FFDl
=
1.
If any of these conditions are met, the OS will attempt to read
the boot record from sector #1 of disk drive 1 and then transfer
control to the software that was read in. The exact sequence of
operations will be,explained later in this section.
Diskette-Boot File Format
The key region of a diskette-boot file is the first six bytes, which
are formatted as shown below:
Figure 10-2
+----------------+
flags first byte
+----------------+
.. of sectors
+----------------+
t memory address t
+-
-+
t
to start load
+----------------+
init
+- -+
address
+----------------+
boot
continuation
code
sixth byte
Diskette-Boot File Format
The first byte is stored in DFLAGS [0240l, but is otherwise
unused. It should equal zero.
The second byte contains the number of 128-byte diskette sectors
to be read as part of the boot process (including the record
containing this information). This number can range from 1 to
255,
with
0
meaning
256.
OPERATING SYSTEM
C016555 --
Section
10
159