12.5 Locating a Constant at a Specific Address in Program Memory

In this example, the constant table is located at a specific address in program memory. When a constant is specifically placed at an address in program memory, it must be placed in its own PSV section using the space(psv) attribute. If a device has only one PSV page (16K instruction words or less), the (psv) section and (auto_psv) section will share the same PSV page by default.

The __builtin_tbladdress() helper function can be used to find the address of a constant stored in program memory. The __psv__ access qualifier is used to specify compiler-managed access.

#include "stdio.h"
#include "xc.h"

__psv__ const unsigned __attribute__ ((space(psv), address (0x2000))) table[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

int main(void)
{
  unsigned sum=0, u;
  long addr;

  /* compute the address of table and print it */
  addr = __builtin_tbladdress(table);

  /* print the address of table */
  printf ("table[] is stored at address 0x%lx\n", addr);

  /* sum the values in table[] */
  for (u=0; u<10; u++) {
       sum += table[u];
  }

  /* print the sum */
  printf ("sum is %d\n", sum);
}

The equivalent constant definition for the array table in assembly language appears below. The .align directive is optional and represents the default alignment in program memory. Use of * as a section name causes the assembler to generate a unique name based on the source file name.

        .section *,address(0x2000),psv
        .global _table
        .align 2
_table:
        .word 0,1,2,3,4,5,6,7,8,9

In order to allocate table in data memory, the space(psv) attribute could be changed to space(data). In this case, the specified address would be a data memory address. In the absence of a space attribute, the keyword const directs the C compiler to allocate the variable in the same space as other compiler constants. Constants are allocated in program memory by default, or in data memory if the constants-in-data memory model is selected.