The big thing was with the 128 was the microprocessor was a MOS 8502 with a 2mhZ option. Since the VIC-II chip was not designed to handle 2mhZ speed, it was turned off when the fast command was used. Also when you turned on the 128 the Z80A chip was in control for the boot process; once the process was completed it switched to the 8502 (explains the short delay when booting).\\ Fun Fact: Bil Herd and his team designed and developed the C128 on a DEC PDP-10 mini computer!