NAME¶

mem_sections - Memory Sections

Remarks

Weak Bindings

The following describes the various sections available.

The .text Section¶

The .text section contains the actual machine instructions which make up your program. This section is further subdivided by the .initN and .finiN sections dicussed below.

Note

The .data Section¶

This section contains static data which was defined in your code. Things like the following would end up in .data:

char err_str[] = "Your program has died a horrible death!";
struct point pt = { 1, 1 };

It is possible to tell the linker the SRAM address of the beginning of the .data section. This is accomplished by adding -Wl,-Tdata,addr to the avr-gcc command used to the link your program. Not that addr must be offset by adding 0x800000 the to real SRAM address so that the linker knows that the address is in the SRAM memory space. Thus, if you want the .data section to start at 0x1100, pass 0x801100 at the address to the linker. [offset explained]

Note

The .bss Section¶

Uninitialized global or static variables end up in the .bss section.

The .eeprom Section¶

This is where eeprom variables are stored.

The .noinit Section¶

This sections is a part of the .bss section. What makes the .noinit section special is that variables which are defined as such:

int foo __attribute__ ((section (".noinit")));

will not be initialized to zero during startup as would normal .bss data.

Only uninitialized variables can be placed in the .noinit section. Thus, the following code will cause avr-gcc to issue an error:

int bar __attribute__ ((section (".noinit"))) = 0xaa;

It is possible to tell the linker explicitly where to place the .noinit section by adding -Wl,--section-start=.noinit=0x802000 to the avr-gcc command line at the linking stage. For example, suppose you wish to place the .noinit section at SRAM address 0x2000:

	$ avr-gcc ... -Wl,--section-start=.noinit=0x802000 ...

Note

Alternatively, you can write your own linker script to automate this. [FIXME: need an example or ref to dox for writing linker scripts.]

The .initN Sections¶

These sections are used to define the startup code from reset up through the start of main(). These all are subparts of the .text section.

The purpose of these sections is to allow for more specific placement of code within your program.

Note

The .initN sections are executed in order from 0 to 9.

.init0:

.init1:

.init2:

.init3:

.init4:

For devices with > 64 KB of ROM, .init4 defines the code which takes care of copying the contents of .data from the flash to SRAM. For all other devices, this code as well as the code to zero out the .bss section is loaded from libgcc.a.

.init5:

.init6:

.init7:

.init8:

.init9:

The .finiN Sections¶

These sections are used to define the exit code executed after return from main() or a call to exit(). These all are subparts of the .text section.

The .finiN sections are executed in descending order from 9 to 0.

.finit9:

.fini8:

.fini7:

.fini6:

.fini5:

.fini4:

.fini3:

.fini2:

.fini1:

.fini0:

The .note.gnu.avr.deviceinfo Section¶

This section contains device specific information picked up from the device header file and compiler builtin macros. The layout conforms to the standard ELF note section layout (http://docs.oracle.com/cd/E23824_01/html/819-0690/chapter6-18048.html).

The section contents are laid out as below.

#define __NOTE_NAME_LEN 4
struct __note_gnu_avr_deviceinfo
{


    struct


    {


        uint32_t namesz;          /* = __NOTE_NAME_LEN */


        uint32_t descsz;          /* =  size of avr_desc */


        uint32_t type;            /* = 1 - no other AVR note types exist */


        char note_name[__NOTE_NAME_LEN]; /* = "AVR\0" */


    }


    note_header;


    struct


    {


        uint32_t flash_start;


        uint32_t flash_size;


        uint32_t sram_start;


        uint32_t sram_size;


        uint32_t eeprom_start;


        uint32_t eeprom_size;


        uint32_t offset_table_size;


        uint32_t offset_table[1];  /* Offset table containing byte offsets into


                                      string table that immediately follows it.


                                      index 0: Device name byte offset


                                     /


        char str_table [2 +


          strlen(__AVR_DEVICE_NAME__)]; /* Standard ELF string table.


                                           index 0 : NULL


                                           index 1 : Device name


                                           index 2 : NULL


                                         /


    }


    avr_desc;
};

Using Sections in Assembler Code¶

Example:

#include <avr/io.h>


    .section .init1,"ax",@progbits


    ldi   r0, 0xff


    out   _SFR_IO_ADDR(PORTB), r0


    out   _SFR_IO_ADDR(DDRB), r0

Note

Using Sections in C Code¶

Example:

#include <avr/io.h>
void my_init_portb (void) __attribute__ ((naked)) \


    __attribute__ ((section (".init3")))


    __attribute__ ((used));
void
my_init_portb (void)
{


    PORTB = 0xff;


    DDRB = 0xff;
}

Note