Added comments.

package/MDAnalysis/coordinates/dcdtimeseries.pyx

@@ -117,12 +117,17 @@ def __read_timecorrel(object self, object atoms, object atomcounts, object forma
             # Check if we have fixed atoms
             # XXX not done
             numskip = step - 1
+            # if the number to skip is greater than the number of frames left
+            # to be jumped over, just take one more step to reflect np slicing
+            # if there is a remainder, guaranteed to have at least one more
+            # frame.
             if(remaining_frames < numskip):
                numskip = 1
 
             rc = skip_dcdstep(dcd.fd, dcd.natoms, dcd.nfixed, dcd.charmm, numskip)
             if (rc < 0):
                 raise IOError("Error skipping frame from DCD file")
+        # on first iteration, numskip = 0, first set is always read.
         dcd.setsread = dcd.setsread + numskip
         rc = read_dcdsubset(dcd.fd, dcd.natoms, lowerb, upperb, tempX, tempY, tempZ, unitcell, dcd.nfixed, dcd.first, dcd.freeind, dcd.fixedcoords, dcd.reverse, dcd.charmm)
         dcd.first = 0

package/MDAnalysis/coordinates/src/dcd.c

@@ -531,12 +531,16 @@ __read_timeseries(PyObject *self, PyObject *args)
       goto error;
    }
 
-   /*Should actually be number of frames, not true yet*/
    remaining_frames = stop-start;
    for (i=0;i<n_frames;i++) {
       if (step > 1 && i>0) {
          // Figure out how many steps to skip
          numskip = step -1;
+         /* If the number to skip is greater than the number of frames left
+            to be jumped over, just take one more step to reflect np slicing
+            if there is a remainder, guaranteed to have at least one more
+            frame.
+         */
          if(remaining_frames < numskip){
             numskip = 1;
          }
@@ -547,6 +551,7 @@ __read_timeseries(PyObject *self, PyObject *args)
             goto error;
          }
       }
+      // on first iteration, numskip == 0, first set is always read.
       dcd->setsread += numskip;
       //now read from subset
       rc = read_dcdsubset(dcd->fd, dcd->natoms, lowerb, upperb, tempX, tempY, tempZ,