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Forum: ARM programming with GCC/GNU tools Casting in C/C++


Casting in C/C++
von Gladson N. (gladson)

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Hi Guys
I have a piece of code for flashing an LED using AT91R40807 on an
AT91EB40 evaluation board.I'm having difficult understanding the
following portion using casting

 #define led_mask = (led1|led2|led3)
 void flashled(int color) {

  *(unsigned int*)(0xFFFOO10) = led_mask
  *(unsigned int*)(0xFFFOO00) = led_mask
  *(unsigned int*)(0xFFFOO34) = led_mask
  ....continues

my question is why were these memory addresses chosen "IN PARTICUALR"
when masking.As far as I know they just represent the location of the
output enable register,enable register and clear output data register
respectively.Probably you can tell me what actually happens with casting
used like this.

G

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Re: Casting in C/C++
von Clifford S. (clifford)

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I am not sure if I understand exactly what it is you are asking; but the
addresses are first cast to pointer types and then derefenced you modify
their content.

The cast is necessary to provide the absolute address with a 'type',
i.e. it says this address points to an 'int'. Without providing type in
this way, the compiler would not know how many bits to write.

Clifford

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