The book "Mapping the 128" by Compute!'s Gazette can probably answer this question in as much detail as you'd ever want, and then some. The descriptions of the KERNAL ROM routines describe the boot sequence in exhaustive detail.
I don't remember the details off the top of my head, but the boot sequence passes through so many indirections and distinct assembly-language stages that I will be genuinely astonished if your problem takes more than a simple JMP (i.e., SYS) to solve.
If you can't readily get hold of a physical copy of the book, there's a pretty good PDFified scan of it on Bombjack.
The world’s only gsteemso
Agitator-in-chief for the Seattle Retro-Computing Society
I actually have the book so I can look at trying to call everything except the thing that auto boots, but I think that it may be harder than it seems since I can't call anything that continues though that routine.
I hope this is the answer you want... it is the best I can give at any rate! Step-by-step:
Hold down the RUN/STOP key (left of SHIFT LOCK key)
Press and release the Reset button (near the main power switch of the computer)
Wait for the "MONITOR" message to appear
Release the RUN/STOP key
Enter the magic command ">A04 C1" (to enable BASIC interrupts)
Enter the well-documented command "X" (to exit the MONITOR and return to BASIC).
Note you can skip step 5 if you don't want/need BASIC interrupts like COLLISION / MOVSPR / PEN / PLAY / SOUND (see my BASIC Encyclopedia of Keywords for details).
Of course if you have questions, please post... and good luck!
I almost forgot! Omitting step 5 can, in some cases, result in BOOTING from device 8, which is what you REALLY do not want (if I understand correctly). So follow all 6 steps to reset the CPU without booting from disk...
Last Edit: Mar 26, 2017 10:58:30 GMT by hydrophilic: Added REBOOT note