Electronic – How does gnu linker obtain Cortex-M stack pointer value

cortex-mgcclinker

GNU's linker obtains the ARM Cortex's stack pointer value from the linker script, and places that value in the first location of the interrupt vector table.

I note that most of my scripts use the symbol _estack to define the "end of stack" (memory region–which is actually the "top of the stack"), which is also the value the linker uses to populate the vector table's MSP location in the final linked image.

Is _estack the (required) symbol the linker uses to generate the MSP? Or, is there another symbol or mechanism GNU's linker uses/can use?

Best Answer

Apparently, there is no special behavior of the GNU linker in obtaining the MSP value: the value is used in the CMSIS-defined vector table, provided by the micro vendor, and defined in the startup file (startup_stm32F303xc.s in my project):

/******************************************************************************
*
* The minimal vector table for a Cortex M4. Note that the proper constructs
* must be placed on this to ensure that it ends up at physical address
* 0x0000.0000.
*
*******************************************************************************/
   .section  .isr_vector,"a",%progbits
  .type  g_pfnVectors, %object
  .size  g_pfnVectors, .-g_pfnVectors


g_pfnVectors:
    .word   _estack
    .word   Reset_Handler
    .word   NMI_Handler
    ...

I had assumed the linker somehow obtained the MSP value (_estack) from the linker script. It is the assembly, however, that simply stubs the value of _estack, which the linker later resolves. Nothing cryptic--just a plain old table of initialized data, placed by the linker at the appropriate address for the microprocessor.

I suppose _estack is simply the canonical symbol used by CMSIS-based projects and linker scripts.

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