Atari 400 Technical Reference Manual page 77

Boot Process Memory Map
The diagram below shows how the boot sector (part of file
DOS.SYS) and following sectors are loaded to memory as part of
the boot process.
Figure 5-13
92
+----------------+
dat,3 .prom boot
=
sector read by
=
: resident OS
+----------------+
data .prom rest
of DOS.SYS
read by the
=
program in the
=
boot sector.
+----------------+
Memory address 0700
077C
077D
end of boot
File Management Subsystem Boot Process Memory Map
OPERATING SYSTEM C016555 -- Section 5