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It is common practice in computer science for the words ‘computer’ and 
‘processor’ to be used interchangeably. More precisely, ‘computer’ refers to the 
central processing unit (CPU) together with an internal memory. The internal 
memory, control and processing components make up the heart of the computer 
system. Manufactures design the CPU to control and carry out basic instructions 
for their particular computer. 
In digital computers the CPU can be divided into two functional units called the 
control unit (CU) and the arithmetical-logical unit (ALU). These two units are 
made up of electronic circuits with millions of switches that can be one of two 
states, either on or off. 
The function of the control unit within the central processor is to transmit 
coordinating control signals and commands. The control unit is that portion of the 
computer that directs the sequence or step-by-step operation of the system, selects 
instructions and data from memory, interprets the program instructions, and 
controls the flow between main storage and the arithmetical-logical unit. 
A control unit has the following components: 
a) a counter that selects the instructions, one at a time, from the memory; 
b) a register that temporarily holds the instruction read from memory while it is 
being executed; 
c) a decoder that takes the called instruction and breaks it down into individual 
commands necessary to carry it out; 
d) a clock, which while not a clock in the sense of a time-keeping device, does 
produce marks at regular intervals. 
This timing marks are electronic and very rapid. 
The arithmetical-logical unit (ALU) is that portion of the computer in which the 
arithmetical operations, namely, addition, subtraction, multiplication, division and 
exponentiation, called for in the instructions are performed. 
Programs and the data on which the control unit and the ALU operate, must be in 
internal memory in order to be processed. Thus, if located on secondary memory 
devices such as disks or tapes, programs and data are first loaded into internal 
memory. 
The primary components of the ALU are banks of bistable devices, which are 
called register. Their purpose is to hold the numbers involved in the calculation 
and hold the result temporarily unit they can be transferred to memory. 
At the core of the arithmetical-logical unit is a very high-speed binary adder, which 
is used to carry out at last the four basic arithmetical functions (addition, 
subtraction, multiplication and division). 

Typical modern computers can perform as many as one hundred thousand 
additions of pairs of thirty-two binary numbers within a second. 
The logical unit consists of electronic circuitry, which compares information and 
makes decisions based upon the result of the comparison. The decisions that can be 
made are whether a number is greater than, equal to, or less than another number. 

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