Re: [avr-gcc-list] Allocating code to separate sections

[Senthil Kumar Selvaraj]

On Thu, Oct 09, 2014 at 08:49:27AM +0200, Alistair Gadd wrote:
> 
> On 2014/10/09 08:29 AM, Senthil Kumar Selvaraj wrote:
> >On Thu, Oct 09, 2014 at 07:32:51AM +0200, Alistair Gadd wrote:
> >>Hi Everybody,
> >>
> >>I'm hoping this is an easy one for someone out there.
> >>
> >>I'm putting together a bootloader for an ATmega328P using Atmel Studio 6.2
> >>and I would just like to ask if there is an easy way configure the IDE so
> >>that I can have separate sections that I can allocate code to. I have been
> >>using "-Ttext 0x7000" as a linker flag in "Configuration
> >>Manager->Toolchain->AVR/GNU Linker->Miscellaneous", which places all the
> >>code starting with reset vector at 0x3800 onwards, but I would like to have
> >>another section in the application section where I can place an SD card
> >>library that will be accessible by the bootloader and by future
> >>applications.
> >>
> >>I have defined another "FLASH segment" - ".sdlib=0x2000" in "Configuration
> >>Manager->Toolchain->AVR/GNU Linker->Memory Settings, but I don't know how to
> >>configure .c source files and assembler .s files so that their code is
> >>allocated to this segment.
> >>
> >>Can anybody give me some ideas on how I could achieve this?
> >There are a couple of ways.
> >
> >1. Create a linker script that groups code from specific object
> >files into custom output sections. Something like
> >
> >.sdlib :
> >{
> >     foo.o(*.text)
> >}
> >
> >where foo.o is the object file containing the code to go into the
> >section.
> >
> >2. Use attributes in C code and .section directives in assembler code to
> >specify the section.
> >
> >void __attribute__((section(".sdlib"))) my_function() {}
> >int main() { return 0; }
> >
> >This makes the place the my_function's code in a .sdlib section. main's
> >code still goes into .text.
> >
> >Hope this helps.
> >
> >Regards
> >Senthil
> Thanks very much Senthil,
> 
> I have tried the __attribute__((section(".sdlib"))) directive which works
> but I'm having problems trying to define variables in flash by using:
> 
> const unsigned char __attribute__((section(".sdlib"))) array[5] PROGMEM  =
> {1,2,3,4,5};
> or:
> const unsigned char array[5] __attribute__((section(".sdlib"))) PROGMEM  =
> {1,2,3,4,5};

The PROGMEM macro expands to a section attribute as well, which is
probably why you're running into problems.
> 
> I think I prefer your first option though because it would put everything in
> that module into the .sdlib section, but my problem is I don't know how to
> create and use the linker script. Can you maybe point me to some
> documentation that would show me how it is used.

https://sourceware.org/binutils/docs/ld/Scripts.html.

For atmega328p, the default linker script is at
<install_prefix>/avr/lib/ldscripts/avr5.xn. With a custom .sdlib
section, it looks like this

/* Script for -n: mix text and data on same page */
OUTPUT_FORMAT("elf32-avr","elf32-avr","elf32-avr")
OUTPUT_ARCH(avr:5)
MEMORY
{
  text   (rx)   : ORIGIN = 0, LENGTH = 128K
  data   (rw!x) : ORIGIN = 0x800060, LENGTH = 0xffa0
  eeprom (rw!x) : ORIGIN = 0x810000, LENGTH = 64K
  fuse      (rw!x) : ORIGIN = 0x820000, LENGTH = 1K
  lock      (rw!x) : ORIGIN = 0x830000, LENGTH = 1K
  signature (rw!x) : ORIGIN = 0x840000, LENGTH = 1K
  user_signatures (rw!x) : ORIGIN = 0x850000, LENGTH = 1K
}
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  .sdlib  : 0x2000
  {
      foo.o(*.text)
      foo.o(*.text.*)
      /* Other object files to go into sdlib */
  }
  /* Internal text space or external memory.  */
  .text   :
  {
    *(.vectors)
    KEEP(*(.vectors))
    /* For data that needs to reside in the lower 64k of progmem.  */
     *(.progmem.gcc*)
    /* PR 13812: Placing the trampolines here gives a better chance
       that they will be in range of the code that uses them.  */
    . = ALIGN(2);
     __trampolines_start = . ;
    /* The jump trampolines for the 16-bit limited relocs will reside here.  */
    *(.trampolines)
     *(.trampolines*)
     __trampolines_end = . ;
     *(.progmem*)
    . = ALIGN(2);
    /* For future tablejump instruction arrays for 3 byte pc devices.
       We don't relax jump/call instructions within these sections.  */
    *(.jumptables)
     *(.jumptables*)
    /* For code that needs to reside in the lower 128k progmem.  */
    *(.lowtext)
     *(.lowtext*)
     __ctors_start = . ;
     *(.ctors)
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     *(.dtors)
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    KEEP(SORT(*)(.dtors))
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       below or above the 16 bits boundary.  */
    *(.init0)  /* Start here after reset.  */
    KEEP (*(.init0))
    *(.init1)
    KEEP (*(.init1))
    *(.init2)  /* Clear __zero_reg__, set up stack pointer.  */
    KEEP (*(.init2))
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Regards
Senthil