saved 4 bytes (modified readfat12)
saved 4 bytes: made getfatpos inline
saved 10 bytes: made searchfile inline
saved 8 bytes: made readfat12 inline
saved 2 bytes: now getrootdirpos breaks %ax
saved 2 bytes: improved readdisk
saved 2 bytes: improved cluster2sector
saved 4 bytes: optimized exit of searchfile