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

[Mark Deneen]

In your example, nothing has been declared.  Types have been defined, and you have a preprocessor macro mapping a particular address to a type, but nothing has been declared.

I don't think that it's doing what you think it is doing.  You're fine in this situation simply because you are accessing peripheral registers and not SRAM.

-M 

----- Original Message -----
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.


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