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RE: [avr-gcc-list] Howto put constants on fixed address

From: Martin Žižka
Subject: RE: [avr-gcc-list] Howto put constants on fixed address
Date: Thu, 20 Dec 2007 00:03:55 +0100

Yes, that will place the .length segment between .vectors and .progmem. But
you are unable to place it to some exact address. That will place it to
address that is offset_of(.vectors)+length_of(.vectors). That can be
different for some cpus.

-----Original Message-----
From: Stu Bell [mailto:address@hidden 
Sent: Wednesday, December 19, 2007 6:26 PM
To: address@hidden; address@hidden
Subject: RE: [avr-gcc-list] Howto put constants on fixed address

I have code in my bootloader that runs a CRC on the loaded code for a sanity
check.  For that I need a length.  Of course, that needs to be supplied
after the code is compiled, but I allocate space for it by doing the

In the .c file:

/* flash code length -- this is loaded in flash by the linker and other
scripts */ const uint32_t ProgramLength __attribute__ ((section
(".length"))) = 0;

Modified linker script (This is avr<architecture>.x in
<WinAVR>\avr\lib\ldscripts - for me avr6.x):

. . . 
  /* Internal text space or external memory.  */
  .text :
    /* For data that needs to reside in the lower 64k of progmem.  */
. . .

(where the .length stuff is what I added)

Give it a shot.

Best regards, 

Stu Bell
DataPlay (DPHI, Inc.) 

-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Sent: Wednesday, December 19, 2007 10:07 AM
To: address@hidden
Subject: RE: [avr-gcc-list] Howto put constants on fixed address

Well, perhaps I've written something wrong, because I think that you are
talking about something else. Or I did not understand that, that is also
possible :-).

I need to put my constants on specific address into flash memory, not
into sram. I want to have the resulting flash content like this:

0000-03FF    some part of .text section (including vectors and so on)
0400-041F    my .foo section
0420-end     the rest of .text section
>> Hi, I need to put some constants on specific address, for example 
>> 0x400.
>> I've declared my constants in a section .foo:
>> char __attribute__ ((section(".foo"))) myconsts="qwertyuiop";
>> then I've compiled that code using:
>> -Wl,-section-start=.foo=0x200
>> But I get this error:
>> .... ld.exe: section .foo [00000400 -> 0000040a] overlaps section 
>> .text [00000000 -> 00001b7d]
>> I've tried to change the linker script and so on, but I was unable to

>> "insert" my code into the .text section. The only solution was to 
>> place my section after the end of the .text section, so the sections 
>> do not overlap, but that is not what I want. Is it possible to put 
>> section to any address?
>> Or are the sections "unbreakable"?
>I thought this answer was in the avr-libc user manual somwhere.
>The linker cannot do separate address spaces, it only knows about a 
>single address space. So .data, .bss, .eeprom are set at specific, very

>high offsets to make them fit in this single address space and not 
>overlap the .text (code) section. Add the relevant offset to your 
>-section-start flag. See the default linker scripts to see how this 

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