CODEWITHSUNDEEP
carraysembeddedconstants
prt
prt

Reputation: 215

how to initialize a const array at specific address in memory?

This question is about embedded controllers. I want to initialize a const array in memory. But while storing this array in memory, I want to store it at a specific location say 0x8000. This way I want to occupy some amount of code memory so that later on during run time I can erase that portion and use it for my own other purpose. Basically I want do this:

const unsigned char dummy_string[] = "This is dummy string";

but the address of dummy_string should be in my hand. Like I can assign whatever address I want to it.

Upvotes: 8

Views: 9022

Answers (3)

kkrambo
kkrambo

Reputation: 7057

Use a pragma statement to place the variable into a named memory section. Then use the linker command script to locate the named memory section at the desired address.

I scanned through some MSP430 documentation and I think it might work something like this...

In the source code use #pragma DATA_SECTION.

#pragma DATA_SECTION(dummy_string, ".my_section")
const unsigned char dummy_string[] = "This is dummy string";

Then in the linker .cmd file do something like this.

MEMORY
{
    ...
    FLASH    : origin = 0x8000, length = 0x3FE0
    ...
}

SECTIONS
{
    ...
    .my_section    : {} > FLASH
    ...
}

If there are multiple sections located in FLASH then perhaps listing .my_section first will guarantee that it is located at the beginning of FLASH. Or maybe you should define a specially named MEMORY region, such as MYFLASH, which will contain only .my_section. Read the linker command manual for more ideas on how to locate sections at specific addresses.

Upvotes: 6

Ben Voigt
Ben Voigt

Reputation: 283893

From C and/or C++, just the way you have written in the question. Possibly add an extern to override C++'s const-is-static-by-default rule.

Then you will need to use a linker directive (.ld file perhaps) to force that symbol to a particular address in code flash/ROM.

Or, you can assume something outside the build process programs the memory, and your code just accesses it. Then you can do something like:

inline const unsigned char* dummy_string() { return (const unsigned char*)0x8000; }

Upvotes: 0

Severin Pappadeux
Severin Pappadeux

Reputation: 20130

Portable way is to use pointer to set address

  const unsigned char dummy_string[] = "This is dummy string";
  unsigned char* p = (unsigned char*)0x1234;

  strcpy(p, dummy_string);

Non-portable way is to use compiler/platform-specific instructions to set address. For example, for GCC on AVR one can use something like

  int data __attribute__((address (0x1234)));

Upvotes: 1

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