Register Storage Class of data types in C programming language

The features of a variable defined to be of register storage class are as under:
Storage
- CPU registers.
Default initial value
- Garbage value.
Scope
- Local to the block in which the variable is defined.
Life
- Till the control remains within the block in which the variable is defined.

A value stored in a CPU register can always be accessed faster than the one that is stored in memory. Therefore, if a variable is used at many places in a program it is better to declare its storage class as register. A good example of frequently used variables is loop counters. We can name their storage class as register.
main( )
{
register int i ;
for ( i = 1 ; i <= 10 ; i++ )
printf ( "\n%d", i ) ;
}
Here, even though we have declared the storage class of i as register, we cannot say for sure that the value of i would be stored in a CPU register. Why? Because the number of CPU registers are limited, and they may be busy doing some other task. What happens in such an event... the variable works as if its storage class is auto.
Not every type of variable can be stored in a CPU register.
For example, if the microprocessor has 16-bit registers then they cannot hold a float value or a double value, which require 4 and 8 bytes respectively. However, if you use the register storage class for a float or a double variable you won’t get any error messages. All that would happen is the compiler would treat the variables to be of auto storage class.