Atari 400 Technical Reference Manual page 145

The third and fourth bytes contain the address (la, hi) at which to
start loading the first bvte of the file.
The fifth and sixth bVtes contain the address (lo,hi) to which the
boater will transfer control after the boot process is complete
and whenever the [SYSTEM.RESET1 key is pressed.
The Diskette File Management System (FMS) has extra bytes assigned to
its boot record, but this is a special case of the generalized
diskette-boot and is discussed in Section 5.
Diskette-Boot Process
If no cartridge is installed, then the diskette will follow these
steps to boot up:
160
1. Read the first diskette record to the cassette buffer [04001.
2. Extract information from the first six bytes:
Save the flags byte to DFLAGS [0240,11. Save the I of sectors
to boot to DBSECT [0241,13. Save the load address to BOOTAD
[0242,23. Save the initialization address in DOSINI [000C,21.
3. Move the record Just read to the load address specified.
4. Read the remaining records directly to the load area.
5. JSR to the load address+6 where a multistage boot process can
continue. The carry bit indicates the success of this
operation (carry set
=
error, carry reset
=
success).
NOTE: During step 5, after the initial boot process is
complete, the booter will transfer control to the seventh byte
of the first record. The software should continue the boot
process at this point, if it is a multistage boot. The value
of MEMLO [02E71 should point to the first free RAM location
beyond the software Just booted. It should be established by
the booted software as shown below:
LDA IEND+1 SET UP LSB.
STA
MEMLO
STA APPMHI
LDA IEND+1/256 SET UP MSa.
STA MEMLO+1
STA APPMHI+1
If the booted software is to take control of the
system at the end of the boot operation, the
vector DOSVEC (OOOAJ must be set up by the
application at this time; DOSVEC points to the
OPERATING SYSTEM C016555 -- Section 10