save const in code mem on specific adr.

Hello !
I want to save a constant factor in the code area on a spezific adress.
How can I do that ??

I used these commands but I got a warning (Warning L15: CODE SPACE MEMORY OVERLAP
FROM 0000H
TO 0000H)...

header file:
extern unsigned char code code_uc_c;

source file:
unsigned char code code_uc_c _at_ 0x0028 = 0x23;

if I use it without = 0x23 it works..

thank you for your help,
florian.