Coding a bootblock that still uses the OS

[earok]

I'm not quite ready to ditch the OS yet - I'm still reliant on some libraries that use graphics library and cia resource - but I've rewritten everything in my project to optionally to use trackdisk.device rather than opening DOS. I've managed to get it running from a custom NDOS disk with a custom boot sector, but -

One issue I'm having is the instructions I've seen for writing boot blocks (http://amigadev.elowar.com/read/ADCD.../node015A.html) say to put the destination in A0 and return - that seems to work fine on A1200, but if I try it on A500 it immediately gurus.

If I don't return from there and just directly jump to the game, I'm assuming it's not freeing as much memory as it could (that documentation mentions that returning from the boot sector cleans up the boot graphic etc).

I'm not quite sure what I'm missing, but until I'm ready to completely ditch the OS, I'd definitely love to be able to recover as much memory as possible.

Edit:
I read https://eab.abime.net/showthread.php?t=110841 and it pointed out that registers need to be preserved, so I preserved everything except the stack, D0 and A0, and it does seem to be booting under A500 settings now. I *think* I can take it from here.