Replacement startup routine for STM32L0XX (or really most any Cortex-M0[+])

reset.s

  .syntax unified
  .cpu cortex-m0plus
  .fpu softvfp
  .thumb

.global  g_pfnVectors
.global  Default_Handler

/* start address for the initialization values of the .data section.
defined in linker script */
.word  _sidata
/* start address for the .data section. defined in linker script */
.word  _sdata
/* end address for the .data section. defined in linker script */
.word  _edata
/* start address for the .bss section. defined in linker script */
.word  _sbss
/* end address for the .bss section. defined in linker script */
.word  _ebss

    .section  .text.Reset_Handler
  .weak  Reset_Handler
  .type  Reset_Handler, %function
Reset_Handler:
/* Don't need to set this here, the stack pointer is loaded out of
 * the vector table automatically by the core on boot.
   ldr   r0, =_estack
   mov   sp, r0
 */

/* Copy the data segment initializers from flash to SRAM */
/*
  movs  r1, #0
  b  LoopCopyDataInit
CopyDataInit:
  ldr  r3, =_sidata
  ldr  r3, [r3, r1]
  str  r3, [r0, r1]
  adds  r1, r1, #4
LoopCopyDataInit:
  ldr  r0, =_sdata
  ldr  r3, =_edata
  adds  r2, r0, r1
  cmp  r2, r3
  bcc  CopyDataInit
*/

CopyDataInitializersStart:
  ldr   r0, =_sdata   /* write to this addr */
  ldr   r1, =_edata   /* until you get to this addr */
  ldr   r2, =_sidata  /* reading from this addr */
  b     CopyDataInitializersEnterLoop
CopyDataInitializersLoop:
  ldmia r2!, {r3}
  stmia r0!, {r3}
CopyDataInitializersEnterLoop:
  cmp   r0, r1
  bcc   CopyDataInitializersLoop

/*
  ldr  r2, =_sbss
  b  LoopFillZerobss
FillZerobss:
  movs  r3, #0
  str  r3, [r2]
  adds r2, r2, #4
LoopFillZerobss:
  ldr  r3, = _ebss
  cmp  r2, r3
  bcc  FillZerobss
*/

WriteZeroToBssStart:
  ldr r0, =_sbss
  ldr r1, =_ebss
  movs r2, #0
  b WriteZeroToBssEnterLoop
WriteZeroToBssLoop:
  stmia r0!, {r2}
WriteZeroToBssEnterLoop:
  cmp r0, r1
  bcc WriteZeroToBssLoop


/* Call the clock system intitialization function.*/
  bl  SystemInit /* void SystemInit (void) */
/* Call static constructors */
    bl __libc_init_array
/* Call the application's entry point.*/
  bl  main