Fighting the GNU as assembler.

For my RTOS experiments, I need vectors to be nicely linked into the binary, which I download to the Cortex M3 target, being an STM32 development board from Olimex.

To start with, I used to put exception vectors in the C source file, in the following way:

/* Vector Table */
unsigned int *vectors[12]
__attribute__ ((section(".vectors")))= {
  (unsigned int *)     START_STACK_TOP,
  (unsigned int *)     boot_rtos,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     0x00000000,
  (unsigned int *)     CM3_SVC
};

That way, I tell gcc to put the array of vectors in the .vectors section. As I use OpenOCD to communicate with the board, using the ARM-USB-OCD, I can download the executable directly from gdb, using the "load" command.

However, I would like to have the vectors defined in an assembly language source file, together with a few of my Cortex M3 specific routines (such as system call handling etc). Therefore I tried to put the vectors in the .s file:

    .section        .vectors,"aw"
    .global    CM3_vectors
    .extern    boot_rtos
CM3_vectors:
        .word   START_STACK_TOP
        .word   boot_rtos
        .word   0
        .word   0
        .word   0
        .word   0
        .word   0
        .word   0
        .word   0
        .word   0
        .word   0
        .word   CM3_handler_svc
        .word   0
        .word   0
        .word   CM3_handler_pendsv

However, I initially did not use the "aw" attribute to the .section operator. This resulted in the vectors area being considered a debug symbol when using nm. Therefore, gdb did not download it to the board.

Using the -S flag to gcc, I could examine the assembly output from gcc and that way find out that I should use the "aw" attribute. Apparently GNU as does not apply any default attributes on a section called .vectors.