Thread: Pointers to Special function registers

Hello everyone,
I am having trouble with pointers, I understand that pointers are just variables that store memory address and here is a example from ARM MCU.

For example there is a special function register called General purpose I/O B Port Data Direction Register or just GPIOB_PDDR.

GPIOB_PDDR has address 0x400FF054 in memory address map and what bothers me is that from a book that I learn ARM MCU author declares variable as

Code:

#define GPIOB_PDDR (*((volatile unsigned int*)0x400FF054))

I understand it this variable has to be type of integer pointer since we store address in it. so part "(volatile unsigned int*)0x400FF054)" is clear to me.

But why do we have another asterix * before the brackets?

Could I declare it something like

Code:

volatile unsigned int* GPIOB_PDDR = 0x400FF054

Also when we write

Code:

GPIOB_PDDR |= 0x80000;

Do we write a value to a register which GPIOB_PDDR aka pointer points to?

> But why do we have another asterix * before the brackets?
So you can write this
GPIOB_PDDR |= 0x80000;
instead of this
*GPIOB_PDDR |= 0x80000;

It's a convenience thing.

> Do we write a value to a register which GPIOB_PDDR aka pointer points to?
Yes, you end up writing to the memory at address 0x400FF054