Porting "far" flash memory access from Arduino Mega to Due

Question

I have a project that runs on the Arduino Mega 2560 which takes up almost the complete amount of flash memory available (https://github.com/fredlcore/bsb_lan/blob/master/BSB_lan.ino). Because of this, I'm planning on migrating the project to the Arduino Due.

My main problem at the moment is to port those functions that access flash memory in the "far" memory area (>64k) for which on the one hand various _PF functions (like strncpy_PF) are provided, and on the other hand a dummy "pointer" type uint_farptr_t (which is actually uint32_t) is available.

My problems now are twofold:

1. While the _PF functions have specific definements for the Due which under "normal" circumstances should work without changes in the code, they expect different variable types. For example, strncpy_PF takes uint_farptr_t (i.e. uint32_t) as argument for src on the Mega, it expects a const char * on the Due. Compilation thus results in lots of errors.

2. For the Mega to be able to access char arrays (which are 16 Bit on the Mega) above 128k, I need a rather dirty workaround (see function calc_enum_offset() in the code linked above). Basically I need to take the memory address of a 16 Bit char pointer and make some calculations to get the final address from which I read the strings. On the Mega, I can do these calculations on the basis of uint_farptr_t and then read from the calculated memory address with pgm_read_byte_far() for example. On the Due, however, pgm_read_byte_far(addr) is translated to (*(const unsigned char *)(addr)). Again, the uint32_t type does not fit this.

Do you have any suggestions on how I should tackle these issues with the least amount of code?

Answer 1

The Due, being ARM, is a von Neumann architecture. That means that it has no concept of "progmem", "near" or "far" pointers, etc. There are no _P or _PF variant functions, but the Arduino core provides simple stubs as a convenience.

Instead all "flash" memory access is just simple memory accesses using const.

The best thing you can do is to use preprocessor macros to select between AVR and ARM code:

#if defined(__AVR__)
    strncpy_PF(buffer, enumstr+c, sizeof(buffer));
    buffer[sizeof(buffer)-1] = 0;
    outBufLen+=sprintf(outBuf+outBufLen,"%d - %s",val,buffer);
#else
    outBufLen += sprintf(outBuf + oufBufLen, "%d - %s", val, cmdtbl[i].enumstr);
#endif

In fact, given the huge amount of "hackiness" involved in that program with regards the cmdtbl, and the massive amounts of code that just aren't needed on the Due, it may be better to start a fresh program that is Due specific and delete the swathes of unneeded code.