Commented the FR_FCFS scheduler

since the original implementation the comments are missed. I just added
the comments such that anybody can understand it.

DRAMSys/simulator/src/controller/scheduler/Fr_Fcfs.cpp

@@ -42,20 +42,43 @@
 
 using namespace std;
 
-
+// The FR_FCFS is descibed in a 2000 paper from Rixner et al.:
+// Memory Access Scheduling
+//
+// The FR_FCFS scheduler features for each bank in the DRAM a specific
+// scheduling buffer for example:
+//
+// Bank0: OOOOOOOO
+// Bank1: OOXXXXXX
+// ...      ^    ^
+// ...      |    |
+// ...   back    |
+// ...       front
+// ...
+// Bank6: OOOOO0XX
+// Bank7: XXXXXXXX
 
 void FR_FCFS::schedule(gp *payload)
 {
-    buffer[DramExtension::getExtension(payload).getBank()].emplace_back(payload);
+
+    // FIXME: Question: what if the buffer is full? IMHO the schedule function
+    // should provide a true or false when the placement into the buffer worked
+    // out or not (?).
+    buffer[DramExtension::getExtension(payload).getBank()]
+            .emplace_back(payload);
 }
 
 std::pair<Command, gp*> FR_FCFS::getNextRequest(Bank bank)
 {
+    // If the bank is empty like Bank0 in the example we do nothing
     if(buffer[bank].empty())
     {
         return pair<Command, tlm::tlm_generic_payload*>(Command::NOP, NULL);
     }
 
+    // In FR_FCFS row hits have always the highest priority, therefore we search
+    // for row hits. If we find a row hit, we remove the transaction from the
+    // queue and send it to the DRAM.
     deque<gp*>::iterator it = FindRowHit(bank);
     if(it != buffer[bank].end())
     {
@@ -64,9 +87,19 @@ std::pair<Command, gp*> FR_FCFS::getNextRequest(Bank bank)
         return pair<Command, gp*>(getReadWriteCommand(*payload), payload);
     }
 
-    return pair<Command, gp*>(getNextCommand(buffer[bank].front()), buffer[bank].front());
+    // If there is no row hit, the FR_FCFS takes always the oldest transaction
+    // in the buffer, i.e. the transaction in the front.
+    return pair<Command, gp*>(getNextCommand(buffer[bank].front()),
+                              buffer[bank].front());
 }
 
+// This function searches for a row hit in the scheduling queue of the specific
+// bank. If no row hit is found the end of the queue is returned.
+//
+// Note: end() Returns an iterator referring to the past-the-end element in the
+// deque container. The past-the-end element is the theoretical element that
+// would follow the last element in the deque container. It does not point to
+// any element, and thus shall not be dereferenced.
 deque<gp*>::iterator FR_FCFS::FindRowHit(Bank bank)
 {
     deque<gp*> &queue = buffer[bank];
@@ -74,14 +107,17 @@ deque<gp*>::iterator FR_FCFS::FindRowHit(Bank bank)
     if(!controllerCore.getRowBufferStates().rowBufferIsOpen(bank))
         return queue.end();
 
+    // Traverse the scheduling queue of the specific bank:
     for(auto it = queue.begin(); it!=queue.end(); it++)
     {
         gp* payload = *it;
-        //Found row-hit
-        if(DramExtension::getRow(payload) == controllerCore.getRowBufferStates().getRowInRowBuffer(bank))
+        //Found row-hit and return the according iterator
+        if(DramExtension::getRow(payload)
+                == controllerCore.getRowBufferStates().getRowInRowBuffer(bank))
         {
             return it;
         }
     }
+
     return queue.end();
 }