diff --git a/tools/ffu_extract.py b/tools/ffu_extract.py
index 52aa0b8..de01ec9 100644
--- a/tools/ffu_extract.py
+++ b/tools/ffu_extract.py
@@ -18,27 +18,25 @@ CHUNK = 131072
 
 
 def load(ffu):
-    f = open(ffu, "rb")
+    import mmap
+    raw = open(ffu, "rb")
+    f = mmap.mmap(raw.fileno(), 0, access=mmap.ACCESS_READ)
     sec = f.read(32)
     _, _, chunk_kb, _, cat, hsh = struct.unpack("<I12sIIII", sec)
     assert chunk_kb * 1024 == CHUNK
-    off = 32 + cat + hsh
-    off += (CHUNK - off % CHUNK) % CHUNK
-    f.seek(off)
-    ih = f.read(24)
-    mlen = struct.unpack("<I", ih[16:20])[0]
-    man = f.read(mlen)
-    st = off + 24 + mlen
-    f.seek(st + 204)
-    vals = struct.unpack("<11I", f.read(44))
+    # store header: ASCII PlatformId inside it; header starts 12B before
+    st = f.find(b"Microsoft.MSM") - 12
+    assert st > 0
+    vals = struct.unpack("<11I", f[st + 204:st + 248])
+    assert vals[0] == CHUNK, vals  # blkSize sanity
     wcnt = vals[1]
     pos = st + 248
     disk = {}   # diskchunk -> datachunk
     dc = 0
     for _ in range(wcnt):
-        f.seek(pos)
-        a, _ = struct.unpack("<II", f.read(8))
-        locs = [struct.unpack("<II", f.read(8)) for _ in range(a)]
+        a, _ = struct.unpack("<II", f[pos:pos + 8])
+        locs = [struct.unpack("<II", f[pos + 8 + k * 8:pos + 16 + k * 8])
+                for k in range(a)]
         pos += 8 + 8 * a
         for m, idx in locs:
             if m == 0: