[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [avr-gcc-list] array of pointers to PROGMEM strings

From: raphael deimel
Subject: Re: [avr-gcc-list] array of pointers to PROGMEM strings
Date: Sun, 17 Nov 2002 12:49:38 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.1) Gecko/20020913 Debian/1.1-1

Thedore A. Roth wrote:

On Sat, 16 Nov 2002, raphael deimel wrote:

:)Hi, I stumbled over a problem. I want to define an array of pointers to :)ascii strings, everything in progmem to save RAM.
:)I thought about defining it like this:
:)PGM_P PROGMEM array[2] = {
:)    "ascii string",
:)    "ascii string"
:)But this would put the ascii strings into ram, right?
:)Does someone know how to define it?

This should be added to the FAQ. ;-) (I'll do it today.)

You need to do something like this:

cat <<EOF > foo.c
#include <avr/io.h>
#include <avr/pgmspace.h>

const char foo[] PROGMEM = "Foo";
const char bar[] PROGMEM = "Bar";
PGM_P array[2] PROGMEM = {

int main (void)
   char buf[32];

   strcpy_P (buf, array[1]);   /* copy bar into buf */

   return 0;

Looking at the disassembly of the resulting object file we see that array in in flash as such:

0000008c <__ctors_end>:
 8c:   46 6f           ori r20, 0xF6   ; 246
 8e:   6f 00           .word   0x006f  ; ????

00000090 <bar>:
 90:   42 61           ori r20, 0x12   ; 18
 92:   72 00           .word   0x0072  ; ????

00000094 <array>:
 94:   8c 00           .word   0x008c  ; ????
 96:   90 00           .word   0x0090  ; ????

foo is at addr 0x008c
bar is at addr 0x0090
array is at addr 0x0094

Then in main() we see this:

   strcpy_P (buf, array[1]);   /* copy bar into buf */
 de:   60 e9           ldi r22, 0x90   ; 144
 e0:   70 e0           ldi r23, 0x00   ; 0
 e2:   ce 01           movw    r24, r28
 e4:   01 96           adiw    r24, 0x01   ; 1
 e6:   0e 94 79 00     call    0xf2

The addr of bar (0x0090) is loaded into the r23:r22 pair which is the second
parameter passed to strcpy_P. The r25:r24 pair is the addr of buf.

Is that what you intended to do?

it does what i need, thx ;) But I'm curious about one thing, can such arrays not be defined without giving each string a name? because this would mean definition of an unnamed string (or whatever this is called) work in RAM, but not in ROM..

Ted Roth

PS: If anyone sees a flaw in this discussion, please point it out so the FAQ entry will be correct.
PSS: If someone has a better way to do this, I'm all ears.

avr-gcc-list at http://avr1.org

avr-gcc-list at http://avr1.org

reply via email to

[Prev in Thread] Current Thread [Next in Thread]