IN:
a0 - ptr to bootblock buffer (PC - 12) (not guaranteed!)
a1 - struct IOStdReq * of the booting device
a3 - struct IOStdReq * (same as a1, but not guaranteed!)
a5 - expansionbase (not guaranteed!)
a6 - execbase
a1 and a6 are guaranteed to contain these values [at least KS 1.2-3.1], since almost all bootloaders rely on them.
OUT:
d0 - zero if bootblock succeeded, else nonzero
a0 - if d0 is zero, a0 is ptr to dos.library resident RT_INIT function that is called later to fire up dos.
