Question

I found this statement in header file that came with a micro controller and have no idea how to evaluate it

#define FIO0SET        (*(volatile unsigned long *)(FIO_BASE_ADDR + 0x18))

I don’t have the foggiest idea what this expression is, and so I have no way to start any meaningful research. Can anyone tell me what this beast is or how to evaluate it?

Answer 1

#define FIO0SET  (*(volatile unsigned long *)(FIO_BASE_ADDR + 0x18))

Go at memory address FIO_BASE_ADDR + 0x18 and retreive the 32-bit (if unsigned long is 32-bit) word stored at that location. FIO0SET is also a modifiable lvalue so you can assign it a value.

Note that the volatile qualifier is used to inform the compiler that the value of the object can change in a way unknown to it. Consider for an example an IO register in input mode: its value can change unexpectedly and the compiler cannot assume its value didn’t change after it was read.

For example:

unsigned long register_value;

register_value = FIO0SET; // store FIO0SET in register_value object

or

FIO0SET = 0x1122334;      // store 0x1122334 value in FIO0SET register