Poniter to const, is there a work around?

Guest · Posted: Tue Jul 26, 2005 7:12 pm

Hi,

I'm attemping to use the PIC18 and have calibrations in ROM and want to access them via a pointer in memory.

Thanks for your help

valemike · Guest

Look at this FAQ:

http://www.ccsinfo.com/faq/?19

Ttelmah · Guest

There are however 'semi workrounds', depending on the chip involved. The simplest, is to not use the const declaration, but instead use the #ROM declaration, and put the block of data at a fixed location in ROM memory, that you save and control. Perhaps the last 256 bytes in a typical system not using ICD. The contents of this block can then be accessed using 'read_program_eeprom' (provided the chip is a 'flash' part, offering this ability).
The alternative, is that the 'label_address' function will return the address in ROM, where a constant is stored, and again this can be used with read_program_eeprom to access the data. This however carries a 'caveat', that normally the data is stored with a 'retrieval header', which contains the code to allow it to be accessed, so the real data is a few bytes latter in the memory.
Chips without read_program_eeprom, cannot really be accessed except by the array method, because on these, the locations do not actually contain the required data, but 'retlw' instructions setup to return the value.

Best Wishes