Re: [avr-gcc-list] Structs in program memory.

[Christian Ludlam]

On 26 Apr Douglas Dotson wrote:

> Might work. I have a table-drive user interface
> that requires lots of initialize structs. The only
> thing I worry about is that the size of the tables
> may eventually exceed the available EEPROM space.
> I'd rather just put them in program memory and
> that way I don't have to worry about hitting the
> wall someday. They are never modified on the fly
> so program memory seems more appropriate.

There's no problem with putting them in program memory, just declare them
with the progmem attribute, ie

#include <avr/pgmspace.h>

struct foo bar PROGMEM = 
{
    1; 2; 3;  
};


Reading them is slightly trickier since you can't do this:

    value = bar.one;
    
instead, you need to do
 
    value = pgm_read_byte(&bar.one);


Another thing to remember is that you can't initialise with literal strings
intended for program memory, you have to define them separately, ie

static const char string[] PROGMEM = "Hello, World!";

struct baz bar PROGMEM =
{
    1; 2; string; 
};

and not

struct baz bar PROGMEM = 
{
    1; 2; "Hello, World!";
};


I wrote a menu system using this a while ago, so it's perfectly doable.
There's probably more information on doing this in the FAQ, and certainly in
the avr-libc manual.


HTH,

-- 
Christian Ludlam
address@hidden