Memory monitoring updates

Split update from read value;
Inhibit interrupts when reading (normally done from loop code);
Don't inhibit interrupts when updating (normally done from interupt code);
Make freeMemory() local and ask for inline code generation.

CommandStation-EX.ino

@@ -126,7 +126,7 @@ void loop()
 // Report any decrease in memory (will automatically trigger on first call)
   static int ramLowWatermark = 32767; // replaced on first loop 
 
-  int freeNow = updateMinimumFreeMemory();
+  int freeNow = minimumFreeMemory();
   if (freeNow < ramLowWatermark)
   {
     ramLowWatermark = freeNow;

DCCEXParser.cpp

@@ -720,7 +720,7 @@ bool DCCEXParser::parseD(Print *stream, int params, int p[])
         return true;
 
     case HASH_KEYWORD_RAM: // <D RAM>
-        StringFormatter::send(stream, F("\nFree memory=%d\n"), freeMemory());
+        StringFormatter::send(stream, F("\nFree memory=%d\n"), minimumFreeMemory());
         break;
 
     case HASH_KEYWORD_ACK: // <D ACK ON/OFF> <D ACK [LIMIT|MIN|MAX] Value>

freeMemory.cpp

@@ -35,26 +35,31 @@ extern char *__malloc_heap_start;
 static volatile int minimum_free_memory = 32767;
 
 
-int freeMemory() {
+static inline int freeMemory() {
   char top;
 #if defined(__arm__)
   return &top - reinterpret_cast<char*>(sbrk(0));
 #elif defined(__AVR__)
   return __brkval ? &top - __brkval : &top - __malloc_heap_start;
 #else
-#error bailed out alredy above
+#error bailed out already above
 #endif
 }
 
 // Update low ram level.  Allow for extra bytes to be specified
 // by estimation or inspection, that may be used by other 
-// called subroutines.
-int updateMinimumFreeMemory(unsigned char extraBytes) {
-  byte sreg_save = SREG;
-  noInterrupts();
+// called subroutines.  Must be called with interrupts disabled.
+//
+void updateMinimumFreeMemory(unsigned char extraBytes) {
   int spare = freeMemory()-extraBytes;
   if (spare < minimum_free_memory) minimum_free_memory = spare;
-  int returnValue = minimum_free_memory;
-  SREG = sreg_save;
-  return returnValue;
+}
+
+// Return low memory value.
+int minimumFreeMemory() {
+  byte sreg_save = SREG;
+  noInterrupts(); // Disable interrupts
+  int retval = minimum_free_memory;
+  SREG = sreg_save; // Restore interrupt state
+  return retval;
 }

freeMemory.h

@@ -20,6 +20,6 @@
 
 #ifndef freeMemory_h
 #define freeMemory_h
-int freeMemory();
-int updateMinimumFreeMemory(unsigned char extraBytes=0);
+void updateMinimumFreeMemory(unsigned char extraBytes=0);
+int minimumFreeMemory();
 #endif