The Microprocessor

The term microprocessor typically refers to the central processing unit (CPU) of a microcomputer, containing the arithmetic logic unit (ALU) and the control units. It is typically implemented on a single LSI chip. This separates the "brains" of the operation from the other units of the computer.

An example of microprocessor architecture. The microprocessor contains the arithmetic logic unit (ALU) and the control unit for a microcomputer. It is connected to memory and I/O by buses which carry information between the units.

Buses: The exchange of information.

Information is transferred between units of the microcomputer by collections of conductors called buses.

There will be one conductor for each bit of information to be passed, e.g., 16 lines for a 16 bit address bus. There will be address, control, and data buses.

Arithmetic Logic Unit

All the arithmetic operations of a microprocessor take place in the arithmetic logic unit (ALU). Using a combination of gates and flip-flops, numbers can be added in less than a microsecond, even in small personal computers. The operation to be performed is specified by signals from the control unit. The data upon which operations are performed can come from memory or an external input. The data may be combined in some way with the contents of the accumulator and the results are typically placed in the accumulator. From there they may be transferred to memory or to an output unit.

The Accumulator

The accumulator is the principal register of the arithmetic logic unit of a microprocessor. Registers are sets of flip-flops which can hold data. The accumulator typically holds the first piece of data for a calculation. If a number from memory is added to that date, the sum replaces the original data in the accumulator. It is the repository for successive results of arithmetic operations, which may then be transferred to memory, to an output device, etc.

Control Unit of Microprocessor

The control unit of a microprocessor directs the operation of the other units by providing timing and control signals. It is the function of the microcomputer to execute programs which are stored in memory in the form of instructions and data. The control unit contains the necessary logic to interpret instructions and to generate the signals necessary for the execution of those instructions. The descriptive words "fetch" and "execute" are used to describe the actions of the control unit. It fetches an instruction by sending and address and a read command to the memory unit. The instruction at that memory address is transferred to the control unit for decoding. It then generates the necessary signals to execute the instruction.