Further question with memory mapped interface

问题

I still have some issues with my c code that deals with an memory mapped device. At the moment I declare the address space for the registers I write as volatile pointer and I write data to them as shown below:

volatile unsigned int *wr_register = (int *) 0x40000000;
volatile unsigned int *c_register =  (int *) 0x40000100;
...

main{

  *wr_register = 0x01234567;

  *c_register = 0x01234567;    
  *(c_register+1) = 0x89abcdef;  

}

This works more or less fine. However, I would like to have specific read and write functions that interact with the memory mapped registers. So ideally, it would look something like this:

const unsigned int wr_register = 0x40000000;
const unsigned int c_register  = 0x40000100;

function write_REG(unsigned int address, int offset, int data)
{
    (unsigned int*) (address + offset) = data;
}

main{

  *write_REG(0x40000000, 0, 0x01234567);

  *write_REG(0x40000100, 0, 0x01234567);
  *write_REG(0x40000100, 1, 0x89abcdef);  

}

I have not tried it out yet to be honest, but I am wondering if somebody could tell me if this is a proper way to do it?

EDIT: Maybe it is of use for someone else, here I have my function and they seem to work. Many thanks for the helpful comments!

void reg_write(unsigned int address, int offset, int data)
{
    *((volatile unsigned int*)address + offset) = data;
}

int reg_read(unsigned int address, int offset)
{
    return(*((volatile unsigned int*)address + offset));
}

Many thanks

回答1:

There are quite a few problems with your code:

I assume you meant void where you wrote function.
You should make the pointer inside the function to be volatile as well.
You should dereference the pointer before writing the data. The * should be inside the function, not at the call site (*write_REG) as it is now - that would be a compile error.
You should add the offset to the pointer, not the address. This is because an offset of 1 is meant to be the next int which could be, say, 4 bytes away, but adding it to the address will only add 1 byte.

Your corrected function should look like this:

void write_REG(unsigned int address, int offset, int data)
{
    *((volatile unsigned int*)address + offset) = data;
}

and you would call it like:

write_REG(0x40000000, 0, 0x01234567);

回答2:

That would be just fine IMHO. I sometimes use macros like:

#define WR_REG     *(volatile unsigned int*)0x40000000

This allows the registers to be used sort of like variables:

WR_REG = 0x12345678;

来源：https://stackoverflow.com/questions/4166201/further-question-with-memory-mapped-interface

标签

memory-mapping