How do you assign a specific memory address to a pointer?
The Special Function Registers in a microcontroller such AVR m128 has fixed addresses, the AVR GCC defines the SFR in the io.h header file, but I want to handle it myself.
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Sure, no problem. You can just assign it directly to a variable:
volatile unsigned int *myPointer = (volatile unsigned int *)0x12345678;
What I usually do is declare a memory-mapped I/O macro:
#define mmio32(x) (*(volatile unsigned long *)(x))
And then define my registers in a header file:
#define SFR_BASE (0xCF800000)
#define SFR_1 (SFR_BASE + 0x0004)
#define SFR_2 (SFR_BASE + 0x0010)
And then use them:
unsigned long registerValue = mmio32(SFR_1); // read
mmio32(SFR2) = 0x85748312; // write
Carl Norum
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2I would have used a generic void pointer, unless the data at that location is an unsigned int. – Sean A.O. Harney Mar 05 '10 at 19:17
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2It's usually a good idea to declare memory-mapped I/O addresses as `volatile`. – Paul R Mar 05 '10 at 19:19
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1@Sean, he wants to access a memory mapped register. You can't dereference a `void` pointer, what use would that be? – Carl Norum Mar 05 '10 at 19:19
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Well, you could make it volatile void * so that they can't access it casually except by using mmio32(). I wouldn't do it that way, perhaps that's what you want? – Southern Hospitality Mar 05 '10 at 21:45