Editing Atari ST Troubleshooting Guide (section)

== Boot Sequence Overview ==

Understanding the ST boot sequence helps pinpoint where a failure occurs. This sequence is the same for the 520ST and 1040ST:

# '''Power-on:''' PSU provides +5V and +12V rails. The 68000 is held in reset by the GLUE chip until voltages stabilise.
# '''Reset release:''' GLUE releases the 68000 RESET line. The CPU reads the initial SSP (Supervisor Stack Pointer) from address $000000 and the initial PC (Program Counter) from address $000004, which points into TOS ROM space ($FC0000–$FEFFFF).
# '''TOS initialisation:''' TOS performs a RAM sizing/test routine. On TOS 1.00–1.04, this is a brief test. The screen remains blank (white or black depending on the monitor type) during this phase.
# '''GEM desktop:''' If all tests pass and no bootable floppy is detected, the GEM desktop appears. If a bootable floppy is present, the boot sector is loaded and executed.

'''Failure at each stage produces characteristic symptoms:'''
* No power LED → PSU failure or blown fuse
* Power LED on but no display → CPU not running, clock failure, or ROM space fault
* White screen (colour monitor) → CPU running but stuck in early TOS code; likely RAM fault, ROM fault, or MMU/GLUE failure
* Green screen with corrupted icons → Partial boot; likely bad RAM chip, video fault, or corrupted TOS
* Desktop appears but crashes → PSU instability, intermittent RAM fault, or software issue