RE: AW: [avr-gcc-list] Pgmspace.h Use

[Dave Hansen]

From: "Haase Bjoern (PT-BEU/EMT) *" <address@hidden>

> address@hidden wrote:
>
> >Hello,
> >
> >for code portability reasons, I would like to avoid to call explicity the
> >function pgm_read_byte() like in the following example :
> >
> >const prog_char array[7] = {0x10,0x20,0x30,0x40,0x50,0x60,0x70};
> >y = pgm_read_byte(&array[2]);
> >// I would like to to keep the syntax y = array[2]; without having the 
> array
> >copied in the SRAM
> >
> >Is there a way to do it ?
> >
> >
> >
> No. The declaration you give above *is* how you put the array in Flash,
> and how you access the array in Flash. GCC requires a new feature patch
> for it to know that the expression, y = array[2], means that array is in
> Flash and to use the correct assembly sequence. There is a person
> working on such a feature, however, it is very non-trivial to implement
> in GCC. And currently there is not a deadline when this work will be
> completed as it being written by a volunteer.
>
> Eric Weddington
>
> IMO, it is not a problem of gcc, it is a problem of the C programming 
> language that lacks
> support for Harvard architectures. In order to add support, you are forced 
> to leave the ANSI standard, IIUC.

There is  ISO/IEC WDTR 18037, which addresses multiple memory spaces 
("Harvard architectures"), fixed-point arithemetic, and hardware IO 
addressing.  I'm not sure of what the status is, but you should (soon? now?) 
be able to support this stuff without leaving the standard.

FWIW, to maintain portability between the two compilers I use, my current 
code has a "common.h" file that all the .c files #include.  One of the 
fetures of common.h is a set of preprocessor directives something like

  #if defined(__GNUC__)
  #  include <avr/pgmspace.h>
  #  define flash const
  #else
  #  define pgm_read_byte(p)   (*((unsigned char flash *)(p)))
  #  define pgm_read_word(p)   (*((unsigned short flash *)(p)))
  #  define PROGMEM
  #endif

So I can define data like

  unsigned char flash array[2] PROGMEM = {3,5};

and access it with something like

  x = pgm_read_byte(array+idx);

Maybe the syntax isn't as pretty, but it works.

Regards,
  -=Dave