Question

I’m doing programming of a softcore processor, Nios II from Altera, below is the code in one of the tutorial, I manage to get the code working by testing it on the hardware (DE2 board), however, I could not understand the code.

#define Switches (volatile char *) 0x0003000
#define LEDs (char *) 0x0003010
void main()
{ while (1)
*LEDs = *Switches;
}

What I know about #define is that, it is either used to define a constant, or a macro, but

1. why in the above code, there are casting like, (char *) 0x0003010, in #define?
2. why the 2 constants, Switches and LEDs act like a variable instead of a constant?

Answer 1

1) why in the above code, there are casting like, (char *) 0x0003010, in #define?

Preprocessor macros are textual replacements. So the code comes out as

while (1) {
  *(char *) 0x0003010 = *(volatile char *) 0x0003000
}

which repeated assigns the contents of the input (switch) mapped at 0x3000 to the output (led) mapped at 0x3010.

2) why the 2 constants, Switches and LEDs act like a variable instead of a constant?

Note that those are pointer. So they always point to the same place (which happens to be a couple of memory mapped IO pins or something similar), but there is no guarantee the the contents of those constant locations are constant, and the * appearing before each preprocessor symbol is the pointer de-reference operator.