What Are Register Variables?

Registers are faster than memories to access. Hence when we declare a variable with a registerkey word, the compiler gets to know that the variable can be put to registers. Now whether the variables will be put into the register indeed or not, that depends on the compiler and the number and size of the registers for the corresponding hardware.

In general, the variables which are to be used with high frequencies (like loop variables) are the perfect choice for declaring as register, because the gain in speed would be considerable in those cases.

It is not allowed to access the address of the register variables as accessing the address of the register is illegal.

It is perfectly all right to declare a pointer as register because a register can always store the address of other variables if the size of the register permits that to do.