Register Storage Class in C

Register modifiers tell the compiler to place the variable in a CPU register instead of RAM. The compiler still can choose not to do that for the sake of optimisation. It's more like a request. While a whole struct can have a register modifier, one field of a struct can't have the register modifier.

int main(void) {
	register int i = 10; // is in one of the CPU registers
	return 0;
}

No Address Rule

You cannot take the address/reference of a register variable, as it has no address while in a register.

int main(void) {
	register int i = 10; // is in one of the CPU registers
	int* ptr_i = &i; // not allowed will cause an error
	return 0;
}

While putting the address in a register works:

int main(void) {
	int i = 10;
	register int* ptr_i = &i; // allowed
	return 0;
}