Introduction of Processor Design

A processor unit is the part of a digital system of a digital computer, that implements the operations in the system. It is comprised of a number of registers and the digital functions that implements arithmetic, logic, shift and transfer micro-operations. The processor unit when combined with a control unit that supervises the sequence of micro-operations, is called a central processor unit or CPU. The simple block diagram of a Processor is given below :

The number of registers in a processor unit may very from just one processor register to as many as 64 registers or more. An operation may be implemented in a processor unit either with a single micro-operation or with a sequence of micro-operations. For example, the multiplication of two binary numbers stored in two registers may be implemented with a combinational circuit that performs the oprations by means of gates. AS soon as the signals propogate through the gates, the product is available and can be transferred to a destination register with a single clock pulse. Alternatively the multiplication operation may be performed with a sequence of add and shift micro-operations. The method chosen for the implementation dictates the amount and type of hardware in the processor unit. The computers implement the operations by means of a sequence of micro-operations. In this way, the processor unit need only have circuits that implement simple. basic micro-operations such as add and shift. Other operations, such as multiplication, division, and floating point arithmetic, are generated in conjunction with the control unit. The control unit is designed to sequence the micro-operations to achieve other operations which are not included in the basic set.

The digital functions that implements the micro-operations on the information stored in processor registers is commonly called an arithmetic logic unit or ALU. To perform a micro-operation, the control routes the source information from registers into the inputs of the ALU. The ALU receives the information from the registers and performs a given operation a specified by the control. The result of the operation is then transferred to a destination register. By definition the ALU is a combinational circuit, thus the entire register transfer operation can be performed during one clock pulse interval. All register-to-register operations, including inter-register transfers, in a typical register unit are performed in one common ALU, otherwise, it would be necessary to duplicate the digital functions for each registers.

A Processor must manipulate not only data but also instruction codes and addresses coming from memory. The register that holds and manipulates the operation code of instructions is considered to be part of the control unit. The registers that hold addresses are sometimes included as part of the processor unit, and the address information is manipulated by the common ALU. In some computers, the registers that hold addresses are connected to a separate bus and the address information is manipulated with the separate digital functions.

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