Does it bypass the 16-sector PROMs?
Like MUFFIN it got it's own RWTS.
How exactly does it work and why does it work on the 16-sector PROM upgraded interface?
Because DOS 3.2 also got it's own RWTS code?
The PROM code is only used during the first two stages of boot, not during normal operation.
In detail DOS boot looks like this:
- After Autostart/CTRL-P/PR#n the PROM code loads track 0/sector 0
- Track 0/Sector 0 uses the PROM code to load sector 0 (again) thru 9 and jumps there.
- This code already got it's own RWTS, independant of the PROM code.
- From there on the whole DOS is loaded using the RWTS loaded in step 3
So BOOT13 just bypasses the first step, handles the second, using his own RWTS, then switching to the code loaded who continues with step 3
Sidenote: The reason why the boot code loads 10 sectors instead of like 13 is based on the state of Woz' development of the Disk II controler, at the time the DOS loader was programmed. Originally Woz used FM encoding and could store 9 256 byte sectors on one track. This was based on the data Shugart supplied with the drives. After some intense measuring he decided that these numbers where rather conservative and he tightened the timing to get 10 sectors per track. At that point Randy Wigginton started DOS development. The second stage should, for performance reason read a whole track and continue from there ons. So 10 sectors it was.
Soon after that decision was finalized Woz switched from FM to GCR to increase capacity by ~33% (10 to 13 sectors). But the boot sequence wasn't changed to reflect this.