The main problem with logic control circuits, was that the circuits were designed for a specific task and could not be used for other control applications. Producing different integrated circuits for each individual piece of equipment is extremely expensive. Engineers realised that if they could produce one circuit, which was versatile enough to be used for many applications, they would make a tremendous saving in cost. It was this approach that led to the development of the microprocessor.
The basic idea was to combine two logic circuits. The control logic controlled the function of the main logic. By supplying signals to the control logic, it was possible to change the way that the main logic circuit operated.
The signals sent to the control logic were called instructions, because they effectively instructed the main logic on how to process the input signals that were "fed" to it. A sequence of instructions is called a program and the first devices developed were called programmable controllers. These devices were soon developed into the first microprocessors.
The microprocessor is a programmable device, like the programmable controller, but it has additional components. The most important of these are the registers and the arithmetic and logic unit (ALU).
A register is simply a memory location inside the microprocessor. Some registers have a specific use and others are for general purpose use. Two particularly important registers are called the address register and the accumulator. Data that is to be processed gets sent to the accumulator and the result is then returned to the accumulator, overwriting the original data. E.g. if you wish to add 1 to a binary number, e.g. 0000 0100, you place this number in the accumulator, perform the addition and the result 0000 0101 appears in the accumulator. The address register holds the address of the memory location that the microprocessor is sending data to or retrieving data from.
The Arithmetic and logic unit carries out the main processing of the data, by performing various mathematical and logic operations on it.