Linker script puts bss into flash - why?

Mike Rosing · ‎2026-05-06

When I'm done compiling, the memory details show

"FLASH";"0x08000000";"";"64 KB"
".ITCMSection";"0x00000000";"0x080002a0";"1.36 KB"
".isr_vector";"0x08000000";"0x08000000";"672 B"
".text";"0x08000810";"0x08000810";"29.19 KB"
".rodata";"0x08007cd0";"0x08007cd0";"84 B"
".ARM";"0x08007d24";"0x08007d24";"8 B"
".init_array";"0x08007d2c";"0x08007d2c";"4 B"
".fini_array";"0x08007d30";"0x08007d30";"4 B"
".data";"0x20000000";"0x08007d34";"144 B"
".bss";"0x20000090";"0x08007dc4";"868 B"
"RW_NONCACHEABLE";"0x200003f4";"0x08007dc4";"36 B"
"._user_heap_stack";"0x20000418";"0x08007dc4";"190.98 KB"

All sections from .bss on should NOT be in FLASH. The linker script has

  /* Used by the startup to initialize data */
  _sidata = LOADADDR(.data);

  /* Initialized data sections into "RAM" Ram type memory */
  .data :
  {
    . = ALIGN(4);
    _sdata = .;        /* create a global symbol at data start */
    *(.data)           /* .data sections */
    *(.data*)          /* .data* sections */
    *(.RamFunc)        /* .RamFunc sections */
    *(.RamFunc*)       /* .RamFunc* sections */

    . = ALIGN(4);
    _edata = .;        /* define a global symbol at data end */

  } >DTCM AT> FLASH

  /* Uninitialized data section into "RAM" Ram type memory */
  . = ALIGN(4);
  .bss (NOLOAD):
  {
    /* This is used by the startup in order to initialize the .bss section */
    _sbss = .;         /* define a global symbol at bss start */
    __bss_start__ = _sbss;
    *(.bss)
    *(.bss*)
    *(COMMON)

    . = ALIGN(4);
    _ebss = . + 0x100;         /* define a global symbol at bss end */
/*    __bss_end__ = _ebss; */
  } > DTCM

  RW_NONCACHEABLE (NOLOAD):
  {
    __NONCACHEABLEBUFFER_BEGIN = .;/* create symbol for start of section */
    KEEP(*(noncacheable_buffer))
    __NONCACHEABLEBUFFER_END = .;  /* create symbol for start of section */
/*  } > RAM_NONCACHEABLEBUFFER*/
  } > DTCM

 */
  /* User_heap_stack section, used to check that there is enough Ram  type memory left */
  ._user_heap_stack (NOLOAD):
  {
    . = ALIGN(8);
    PROVIDE ( end = . );
    PROVIDE ( _end = . );
/*    . = . + _Min_Heap_Size;
    . = . + _Min_Stack_Size; */
    . = ABSOLUTE(_estack);
    . = ALIGN(8);
  } > DTCM

The fixed data should go into flash, but the rest of it should not. Why is the linker adding those sections to flash?

Mike Rosing · ‎2026-05-06

The problem was assuming "." is reset with section assignment. I had to add

  . = ORIGIN(DTCM) + SIZEOF(.data);

before the .bss section. Now the linker is not trying to load the flash with empty space (and attempt to consume 3* as much memory as actually exists!)