It has many support devices like ROM, Read-Write memory, Serial interface, Timer, I/O ports etc. All these support devices are interfaced to microprocessor via a system bus. All support devices in a microprocessor based system are external. The system bus is composed of an address bus, data bus and control bus.
All the support devices like Read only memory, Read – Write memory, Timer, Serial interface, I/O ports are internal. There is no need of interfacing these support devices and this saves a lot of time for the individual who creates the system. A microcontroller is nothing but a microprocessor system with all support devices integrated inside a single chip