Changes
/* Booting CPM */
* iobyte (0003) is set to 81
* the current drive is written to &0004. A = 0, B = 1, D=3 etc.
* ad33 'enter firmware' (&ad33) jp is setup.
* the XDPB is queried to find the location of the boot sector
* the boot sector is loaded to &100.
