View source for Putting data in flash on the Arduino

Since main RAM memory is very limited on the Arduino, you need to put const data into flash memory wherever possible.

There is a necessary trick to doing this:

== Declaring the data ==

The 'PROGMEM' macro places an object into program memory (ie flash):

  #include <avr/pgmspace.h>
  ...
  const char myArray [] PROGMEM = { ... } ;

== Accessing the data ==

But to access the data, you have to use a special set of functions:

  pgm_read_byte ( & myArray [ 10 ] ) ;

...there is also 'pgm_read_word' for 16 bit data and 'pgm_read_dword' for 32 bits.  This only works for 16 bit addresses - but until we have Arduino's with more than 64kb of flash, this is not a problem.  For machines with more, there is pgm_read_xxx_far and pgm_read_xxx_near for 32 and 16 bit addresses.  You really need to cast the results of the pgm_read_* routines because they are implemented in machine-code and the return type is kinda ill-defined!