Added linker line in text section for external object files.

src/linker_script.ld

@@ -1,11 +1,27 @@
+/* Entry Point of program, interrupt vector table in this case.*/
+/* IVT: data structure that associates list of interrupt handlers with
+ * a list of interrupt requests in a table of interrupt vectors.
+ */
 ENTRY( InterruptVector )
+
+/*
+ * Memory Layout:
+ * Flash(read execute): 
+ * Ram(read write execute)
+ */
 MEMORY
 {
  FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 16K
  RAM (xrw) : ORIGIN = 0x20000000, LENGTH = 2K
 }
+
+
 SECTIONS
 {
+    /*
+     * Initialization section:
+     * handles startup initialization routines stored in flash.
+     */
     .init :
     {
       _sinit = .;
@@ -14,6 +30,12 @@ SECTIONS
       . = ALIGN(4);
       _einit = .;
     } >FLASH AT>FLASH
+
+    /*
+     * Text Section:
+     * Contains executable code and read-only data(rodata)
+     * Stored in flash. 
+     */
     .text :
     {
       . = ALIGN(4);
@@ -22,15 +44,31 @@ SECTIONS
       *(.rodata)
       *(.rodata*)
       *(.gnu.linkonce.t.*)
+    
+      /* Include additional object files. */
+      *(EXTERN(*)) /* Link against external symbols */
+
       . = ALIGN(4);
     } >FLASH AT>FLASH
+
+    /*
+     * Finalization Section:
+     * Contains finalization routines, executed before exiting the program.
+     */
     .fini :
     {
       KEEP(*(SORT_NONE(.fini)))
       . = ALIGN(4);
     } >FLASH AT>FLASH
+
+    /*PROVODE: defines a symbol only if it is not already defined elsewhere.*/
     PROVIDE( _etext = . );
     PROVIDE( _eitcm = . );
+
+    /*
+     * The preinit_array and init_array handle static/global constructors and 
+     * initalizers.
+     */
     .preinit_array :
     {
       PROVIDE_HIDDEN (__preinit_array_start = .);
@@ -51,6 +89,8 @@ SECTIONS
       KEEP (*(.fini_array EXCLUDE_FILE (*crtbegin.o *crtbegin?.o *crtend.o *crtend?.o ) .dtors))
       PROVIDE_HIDDEN (__fini_array_end = .);
     } >FLASH AT>FLASH
+
+    /* Constructors Section: needed for C++ */
     .ctors :
     {
       KEEP (*crtbegin.o(.ctors))
@@ -59,6 +99,8 @@ SECTIONS
       KEEP (*(SORT(.ctors.*)))
       KEEP (*(.ctors))
     } >FLASH AT>FLASH
+
+    /* Destructors Section: needed for C++ */
     .dtors :
     {
       KEEP (*crtbegin.o(.dtors))
@@ -67,16 +109,27 @@ SECTIONS
       KEEP (*(SORT(.dtors.*)))
       KEEP (*(.dtors))
     } >FLASH AT>FLASH
+
+
+    /* Likely for data alignment.*/
     .dalign :
     {
       . = ALIGN(4);
       PROVIDE(_data_vma = .);
     } >RAM AT>FLASH
+
+    /* Likely for  dynamic loader alignment.*/
     .dlalign :
     {
       . = ALIGN(4);
       PROVIDE(_data_lma = .);
     } >FLASH AT>FLASH
+
+    /*
+     * Data section: Initialized Variables
+     * contains initialized variables copied from flash to ram at startup.
+     * `_sdata` and `_edata` mark the start and end of the `.data` section.
+     */
     .data :
     {
       . = ALIGN(4);
@@ -97,6 +150,11 @@ SECTIONS
       . = ALIGN(4);
       PROVIDE( _edata = .);
     } >RAM AT>FLASH
+
+    /* BSS Section: Contains uninitialized variables.
+     * Holds uninited global/static vars. The runtime must zero out this
+     * section before main function.
+     */
     .bss :
     {
       . = ALIGN(4);