Added support for .d71 images

uc1541

@@ -311,6 +311,57 @@ class D71(Disk):
     Implement d71 directory reader
     """
 
+    def _get_offset(self, track, sector):
+        """
+        Return offset (in bytes) for specified track and sector.
+
+            Track       Sec/trk   # Tracks
+        --------------  -------   ---------
+         1-17 (side 0)    21         17
+        18-24 (side 0)    19          7
+        25-30 (side 0)    18          6
+        31-35 (side 0)    17          5
+        36-52 (side 1)    21         17
+        53-59 (side 1)    19          7
+        60-65 (side 1)    18          6
+        66-70 (side 1)    17          5
+        """
+        offset = 0
+        truncate_track = 0
+
+        if track > 17:
+            offset = 17 * 21 * SECLEN
+            truncate_track = 17
+
+        if track > 24:
+            offset += 7 * 19 * SECLEN
+            truncate_track = 24
+
+        if track > 30:
+            offset += 6 * 18 * SECLEN
+            truncate_track = 30
+
+        if track > 35:
+            offset += 5 * 17 * SECLEN
+            truncate_track = 35
+
+        if track > 52:
+            offset = 17 * 21 * SECLEN
+            truncate_track = 17
+
+        if track > 59:
+            offset += 7 * 19 * SECLEN
+            truncate_track = 24
+
+        if track > 65:
+            offset += 6 * 18 * SECLEN
+            truncate_track = 30
+
+        track = track - truncate_track
+        offset += track * sector * SECLEN
+
+        return offset
+
 
 class D81(Disk):
     """