Thanks Erik and John,
It seems the option value -m is 20 by default.
/sys/src/9/boot/local.c in Plan9:
run("/boot/fossil", "-m", "20", "-f", partition,
"-c", "srv -A fboot", "-c", "srv -p fscons", nil);I have looked contents in /boot/bootfs.paq of 9front. I like the booting process: flexible, elegant and probably powerful.
