add Adafruit Feather M0 (SAMD21G18A), refactor selective content

RamUtils.cpp

@@ -7,13 +7,24 @@
 
 #include <Arduino.h>
 #include <RamUtils.h>
+
 #if defined (__arm__) && defined (__SAM3X8E__) // Arduino Due
 #include <malloc.h>
 #include <stdlib.h>
 #include <stdio.h>
+extern char* _sbrk(int i);
+#endif
+
+#if defined (ARDUINO_ARCH_SAMD) && defined (__SAMD21G18A__) // Adafruit Feather M0
+extern "C" char *sbrk(int i);
+#endif
+
+#if defined (__AVR__) // AVR
+extern "C" int __bss_end;
+extern "C" int *__brkval;
 #endif
 
-/*!
+/**
  * Returns the free ram memory space.
  * @return Free Memory space in Bytes
  */
@@ -22,14 +33,13 @@ int RamUtils::getFreeRam()
   int free_memory = 0;
 #if defined (__arm__) && defined (__SAM3X8E__) // Arduino Due
   struct mallinfo mi=mallinfo();
-  extern char* _sbrk(int i);
   char* heapend = _sbrk(0);
   register char * stack_ptr asm("sp");
   free_memory = stack_ptr - heapend + mi.fordblks;
-  return free_memory;
-#else // AVR
-  extern int __bss_end;
-  extern int *__brkval;
+#elif defined (ARDUINO_ARCH_SAMD) && defined (__SAMD21G18A__) // Adafruit Feather M0
+  char stack_dummy = 0;
+  free_memory = &stack_dummy - sbrk(0);
+#elif defined (__AVR__) // AVR
   if ((int) __brkval == 0)
   {
     free_memory = ((int) &free_memory) - ((int) &__bss_end);
@@ -38,6 +48,6 @@ int RamUtils::getFreeRam()
   {
     free_memory = ((int) &free_memory) - ((int) __brkval);
   }
-  return free_memory;
 #endif
+  return free_memory;
 }