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The central processing unit

The central processing unit (CPU) regulates the operation go tall system components and perfo

arithmetic and logical operations on the data. To accommodate these functions, the cpu consis

two operating units.

1. control unit

2. arithmetic-logic unit

The control unit co ordinates the various operations specifies by the programme instructions. T

operations include receiving data which enter the computer and deciding how and when the da

should be processed. The control unit directs the operation of the arithematic-logic unit. It sendto the ALU and tells the ALU what functions to perform on the data and where to store the resu

The capability of the control unit to accomplish these operations is provided by a set of instruct

called an executive programme which is stored in memory.

Both the control unit and the arithematic-logic unit perform their functions by utilizing register

Computer registers are small memory devices that can receive, hold and transfer data. Each regconsists of binary cells to hold bits of data. The number of bits in the register establishes the w

length the computer is capable of handling.

The arrangement of these registers constitutes several functional areas of the CPU. To accompgiven sequence of programmed instructions, the functions of these register units be as follows

Programme counter: The programme counter holds the location of address of next instruction

instruction word contains two parts: an operator and an operand or a reference to an operand. T

operator defines the type of arithematic or logical operation to be carried out. The operand ususpecifies the data on which the operation is to be performed. The cpu sequences the instruction

performed by fetching word from memmory according to the contends in the programme coun

Memmory address register: The location of data contained in the computers memmory unit m

identifies for an instruction and this is the function of memmory address register. This unit is u

hold the address of data held in memmory address register

Programme counter

Memmory address register

Instruction register

Accumulator

Status register

Arithematic-logic unit

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Instruction register: The instruction register is used to hold the instruction for decoding. Deco

refers to the interpretation of the code instruction word so that the desired operation is carried o

the cpu

Accumulator: An accumulator is a temporary storage register used during an arithematic or lo

operation. For example, in adding two numbers, the accumulator would be used to store the firnumber while the second number was fetched. The second number would then be added to the

The sum still contained in the accumulator, would then be operated on or transferred to a temp

storage, according to the next instruction in the programme.

Status register: Status registers are used to indicate the internal condition of the cpu. A status

is a 1-bit register. Flags are used to identify such conditions as logical decision outcomes, overand interrupt conditions.

Arithematic-logic unit: The ALU provides the circuitry required to perform the various calcu

and manipulations of data. The unit has two inputs for data, inputs for defining the functions toperformed, data outputs and status outputs used to set the status registers or flags

The arithematic-logic unit can be a simple adder or its circuitry can be more complex for perfoother calculations such as multiplication and division. ALUs with simpler circuits are capable

being programmed to perform these more complicated operations, but more computing time is

required. The more complex arithematic-logic units are faster, but these units are more costly.