Introduction to Central Processing Unit | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Central Processing Unit

• The part of the computer that performs the bulk of data processing operations is called the Central Processing Unit (CPU) and is the central component of a digital computer. 
• Its purpose is to interpret instruction cycles received from memory and perform arithmetic, logic and control operations with data stored in internal register, memory words and I/O interface units. 
• A CPU is usually divided into two parts namely processor unit (Register Unit and Arithmetic Logic Unit) and control unit.

Processor Unit

• The processor unit consists of arithmetic unit, logic unit, a number of registers and internal buses that provides data path for transfer of information between register and arithmetic logic unit. The block diagram of processor unit is shown in figure below where all registers are connected through common buses. The registers communicate each other not only for direct data transfer but also while performing various micro-operations.
• Here two sets of multiplexers select register which perform input data for ALU. A decoder selects destination register by enabling its load input. The function select in ALU determines the particular operation that to be performed.

For an example to perform the operation:

• MUX A selector (SELA): to place the content of R1 into bus A.
• MUX B selector (SELB): to place the content of R2 into bus B.
• ALU operation selector (OPR): to provide arithmetic addition A + B.
• Decoder destination selector (SELD): to transfer the content of the output bus into R3.

Control Unit

The control unit is the heart of CPU. It consists of a program counter, instruction register, timing and control logic. The control logic may be either hardwired or micro-programmed. If it is a hardwired, register decodes and a set of gates are connected to provide the logic that determines the action required to execute various instructions. A micro-programmed control unit uses a control memory to store micro instructions and a sequence to determine the order by which the instructions are read from control memory.

The control unit decides what the instructions mean and directs the necessary data to be moved from memory to ALU. Control unit must communicate with both ALU and main memory and coordinates all activities of processor unit, peripheral devices and storage devices. It can be characterized on the basis of design and implementation by:

• Defining basic elements of the processor
• Describing the micro-operation that processor performs
• Determining the function that the control unit must perform to cause the micro-operations to be performed.

Control unit must have inputs that allow determining the state of system and outputs that allow controlling the behavior of system.

The input to control unit are:

• Flag: flags are headed to determine the status of processor and outcome of previous ALU operation.
• Clock: All micro-operations are performed within each clock pulse. This clock pulse is also called as processor cycle time or clock cycle time.
• Instruction Register: The op-code of instruction determines which micro-operation to perform during execution cycle.
• Control signal from control bus: The control bus portion of system bus provides interrupt, acknowledgement signals to control unit.

The outputs from control unit are:

• Control signal within processor: These signals causes data transfer between registers, activate ALU functions.
• Control signal to control bus: These are signals to memory and I/O module. All these control signals are applied directly as binary inputs to individual logic gate.