startup.s file _bss_start and _bss_end

[lilzz]

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.extern __bss_start
.extern __bss_end

ldr r0, =__bss_start
ldr r1, =__bss_end

what are the actual values of _bss_start and _bss_end?
They are not given specific values in startup.s file

[Paeryn]

The linker provides those symbols as defined in the linker script (ld usually provides a default script if you don't explicitly set one).

Part of your link script will collate all the BSS sections of all the object files and set the symbols appropriately.
Normally __bss_start will point to the first free address after the data section and __bss_end will point to the location __bss_start + sizeof(bss section).

Run

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ld --verbose

and you'll see the default linker script (it's quite long). The bss section is in here...

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  .data           :
  {
    PROVIDE (__data_start = .);
    *(.data .data.* .gnu.linkonce.d.*)
    SORT(CONSTRUCTORS)
  }
  .data1          : { *(.data1) }
  _edata = .; PROVIDE (edata = .);
  . = .;
  __bss_start = .;
  __bss_start__ = .;
  .bss            :
  {
   *(.dynbss)
   *(.bss .bss.* .gnu.linkonce.b.*)
   *(COMMON)
   /* Align here to ensure that the .bss section occupies space up to
      _end.  Align after .bss to ensure correct alignment even if the
      .bss section disappears because there are no input sections.
      FIXME: Why do we need it? When there is no .bss section, we don't
      pad the .data section.  */
   . = ALIGN(. != 0 ? 32 / 8 : 1);
  }
  _bss_end__ = . ; __bss_end__ = . ;
  . = ALIGN(32 / 8);
  . = SEGMENT_START("ldata-segment", .);
  . = ALIGN(32 / 8);
  __end__ = . ;
  _end = .; PROVIDE (end = .);
  . = DATA_SEGMENT_END (.);

Google ld linker scripts for info on how scripts work (you can read the manpages for ld but it doesn't go into any detail on how scripts work).