Utilizing Program Flash and Data RAM for Constant Data in C

There are many advantages and one disadvantage if constant data in C is flashed to program flash memory (pROM) and copied to data flash memory (xRAM) at startup, with the usual pROM-to-xRAM initialization.

The advantages are:

constant data is defined and addressed conventionally via C language
pROM flash space is used for constant data (pROM is usually larger than xROM)
the pROM flash is now freed up or available

The disadvantage is that the xRAM is consumed for constant data at run-time.

If you wish to store constant data in program flash memory and have it handled by the pROM-to-xRAM startup process, a simple change is necessary to the pROM-to-xRAM LCF. Simply, place the constant data references into the data_in_p_flash_ROM section after the __xRAM_data_start variable like the other data references and remove the "data in xROM" section. See the following listing.

Listing: Using typical pROM-to-xRAM LCF

.data_in_p_flash_ROM : AT(__pROM_data_start)

 {

   __xRAM_data_start = .;

   * (.const.data.char) # move constant data references here

   * (.const.char)



   * (.data.char) 

   * (.data) 

   

   etc.