[arm-gnu] Placing variable at absolute address in RAM

[Moses McKnight]

Actually, I think it does.  That example just reserved 4 bytes 
(sizeof(int)), but you can just as easily use a struct or probably even 
an array.

Here's an example from the LPC17xx.h header from and NXP cortex-M3 chip.

typedef struct
{
   volatile uint32_t mask[512];
} LPC_CANAF_RAM_TypeDef;

#define LPC_APB0_BASE         (0x40000000UL)
#define LPC_CANAF_RAM_BASE    (LPC_APB0_BASE + 0x38000)
#define LPC_CANAF_RAM  ((LPC_CANAF_RAM_TypeDef *) LPC_CANAF_RAM_BASE)

See the core_cm3.h file from CMSIS for more examples.

If I'm not mistaken when you declare a variable the compiler/linker 
always reserves enough memory for that type - even when you specify the 
starting address like this.

On 04/05/2012 11:40 AM, JM wrote:
> I have come across that one a few times, but it doesn't reserve memory.
>
> ------------------------------------------------------------------------
> *From:* Moses McKnight <moses at texband.net>
> *To:* JM <hondgm at yahoo.com>
> *Sent:* Thursday, April 5, 2012 11:45 AM
> *Subject:* Re: [arm-gnu] Placing variable at absolute address in RAM
>
> with a quick search I came up with the following link (among many others):
> http://stackoverflow.com/questions/4067811/how-to-place-a-variable-at-a-given-absolute-address-in-memory-with-gcc
>
> This is the way it is done in the CMSIS headers and in the device
> headers for the LPC chips I'm using.
>
> Moses
>
> On 04/05/2012 09:37 AM, JM wrote:
>  > The only reason I mentioned about GCC not making it easy is that
>  > apparently at least one compiler makes it easy. I'm so jealous!
>  >
>  > With the RealView ARM C Compiler, you may use either pointer definitions
>  > (as shown above) or *__attribute__((at(/address/)))* keyword to define a
>  > variable at a fixed memory address. In contrast to the pointer
>  > construct, the following definition also makes a correct memory
>  > reservation, so that the area cannot be used twice.
>  >
>  > int var __attribute__((at(0x40001000)));