Before we take a detailed look about the Motorola 68HC11 series of micro controllers it is useful to define what is meant by a micro controller. As the name suggests a micro controller is a small totally packaged controller capable of controlling motor, sensors etc. To appreciate the difference between a micro controller and a personal computer, let us consider a typical functional block of a small computer system. Typical all computer systems consist of four functional blocks: - Input, Output, process and memory as shown below

These blocks handle the means by which information (data) is transferred and processed to achieve a given task (program). Interfacing to the 'real' world is handled by the input/output blocks (IO), data and program is stored in memory and the process computes/performs the required function. In a typical computer these blocks are physical different integrated circuit s and are interconnected by PCB tracks which form a highway bus structure. This independence of the functions allows designer the ability to expand on the design and add additional devices i.e. Disk drives, CD ROMs etc. In the field of micro controllers, most systems simply require a limited (Fixed) amount of I/O and memory; therefore various manufacturers have compressed the entire system onto a single integrated circuit (embedded) and produced a small (micro) controller. There are various examples of micro controllers; each with different basic blocks e.g. Motorola 6805/11, Intel 8031/51, and Microchip PIC16C5X. Each CPU above has its own language and in order to program the engineer has to learn the particular language of his/her computer. Within software engineering the task of programming in machine code (LLL) proved to be very time consuming and a range of compilers were designed to allow the programmer to write a program in HLL and convert it to machine code instructions. Most programs are now coded in a high level language and compiled to the particular LLL required. However for speed/space critical programs, some programs may still have to be encoded in machine code. There are many HLL available e.g. FORTRAN, Pascal, Basic and the most common 'C' and 'C++'. The program is usually stored in the micro-controller, although the microcontrollers are often expandable to allow external EPROM.