diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c18dd8d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+__pycache__/
diff --git a/README.md b/README.md
index bfac4e1..4fd3cda 100644
--- a/README.md
+++ b/README.md
@@ -1,11 +1,11 @@
 GCD Parser
 ==========
 
-This is a parser for GCD files (firmware updates from a well-known manufacturer).
+This is a parser and some tools for working with GCD files (firmware updates from a well-known manufacturer).
 
 It's in Python, so feel free to add cool new features and submit pull requests.
 
-Thanks to TurboCCC and kunix for your work.
+Thanks to TurboCCC, kunix and Alex W. for your work.
 
 Most info from:
 
diff --git a/binsum.py b/binsum.py
new file mode 100644
index 0000000..83c81e3
--- /dev/null
+++ b/binsum.py
@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+# Many thanks to Alex W. who figured this out!
+
+"""
+Calculates the SHA1 of a fw_all.bin until the ending marker.
+"""
+
+from hashlib import sha1
+from struct import unpack
+from grmn import devices
+import sys
+
+FILE = sys.argv[1]
+BLOCKSIZE = 4096
+
+END_MARKER = b"\xff\xff\x5a\xa5\xff\xff\xff\xff"
+
+first_block = True
+past_end = False
+trailer = bytes()
+trailer_pos = -1
+
+csum = sha1()
+print("Reading {} ...".format(FILE))
+with open(FILE, "rb") as f:
+    while True:
+        block = f.read(BLOCKSIZE)
+        if first_block:
+            start = block.find(b"\xff\xff\xff\xff\xf0\xb9\x9d\x38\x0f\x46\x62\xc7")
+            if start < 0:
+                print("No Fenix firmware header found.")
+            else:
+                start += 4
+                hwid = unpack("<H", block[start+24:start+24+2])[0]
+                fver = unpack("<H", block[start+28:start+28+2])[0]
+                print("- Hardware ID: 0x{:04x} / {:d} ({})".format(hwid, hwid, devices.DEVICES.get(hwid, "Unknown device")))
+                print("- Firmware Version: 0x{:04x} / {:04d}".format(fver, fver))
+            first_block = False
+        if END_MARKER in block:
+            end_pos = block.find(END_MARKER)
+            marker_end = end_pos + len(END_MARKER)
+            past_end = True
+            csum.update(block[0:marker_end])
+            block = block[marker_end:]
+            trailer_pos = f.tell() - len(block)
+        if past_end:
+            trailer += block
+        else:
+            csum.update(block)
+        if len(block) < BLOCKSIZE:
+            break
+    f.close()
+print("Calculated SHA1: {}".format(csum.hexdigest()))
+print("SHA1 in file   : {} (offset 0x{:x})".format(trailer[:20].hex(), trailer_pos))
diff --git a/gcd.py b/gcd_old.py
similarity index 95%
rename from gcd.py
rename to gcd_old.py
index fa1e4dc..a480de3 100644
--- a/gcd.py
+++ b/gcd_old.py
@@ -7,7 +7,7 @@ GCD_SIG = b"GARMINd\00"
 
 from binascii import hexlify
 from struct import unpack
-from grmn import ChkSum
+from grmn import ChkSum, devices
 import sys
 
 FILE = sys.argv[1]
@@ -28,12 +28,6 @@ TLV_TYPES = {
     0xffff: "EOF marker",
 }
 
-DEV_TYPES = {
-    1551: "fenix/D2/tactix",
-    2900: "fenix 5 Plus",
-    3196: "D2 Delta",
-}
-
 cksum = ChkSum()
 all_cksum_ok = True
 last_type6_fids = []
@@ -46,12 +40,6 @@ def get_tlv_comment(ttype):
     else:
         return "Type {:04x} / {:d}".format(ttype, ttype)
 
-def get_device(hwid):
-    if hwid in DEV_TYPES:
-        return DEV_TYPES[hwid]
-    else:
-        return "Unknown"
-
 print("Opening {}".format(FILE))
 
 def parseTLVheader(hdr):
@@ -125,7 +113,7 @@ def parseTLV7(payload):
         fid = last_type6_fids[i]
         fdesc = last_type6_fields[i]
         if fid == 0x1009:
-            print("  - {:>20}: 0x{:04x} / {:d} ({})".format(fdesc, v, v, get_device(v)))
+            print("  - {:>20}: 0x{:04x} / {:d} ({})".format(fdesc, v, v, devices.DEVICES.get(v, "Unknown device")))
         elif fid == 0x2015:
             print("  - {:>20}: {} Bytes".format(fdesc, v))
         else:
@@ -163,7 +151,7 @@ with open(FILE, "rb") as f:
         elif ttype == 0x02bd and not fw_all_done:
             hw_id = unpack("H", payload[0x208:0x20a])[0]
             fw_ver = unpack("H", payload[0x20c:0x20e])[0]
-            print("  - Device ID: {:04x} / {:d} ({})".format(hw_id, hw_id, get_device(hw_id)))
+            print("  - Device ID: {:04x} / {:d} ({})".format(hw_id, hw_id, devices.DEVICES.get(hw_id, "Unknown device")))
             print("  - Firmware version: {:04x} / {:d}".format(fw_ver, fw_ver))
             fw_all_done = True
         else:
diff --git a/gcdstruct.py b/gcdstruct.py
new file mode 100644
index 0000000..eff8883
--- /dev/null
+++ b/gcdstruct.py
@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""
+Prints out the structure of the given GCD file.
+"""
+
+from grmn import Gcd, ChkSum
+import sys
+
+FILE = sys.argv[1]
+
+print("Opening {}".format(FILE))
+
+gcd = Gcd(FILE)
+
+for i, tlv in enumerate(gcd.struct):
+    print("#{:03d}: {}".format(i, tlv))
diff --git a/gcksum.py b/gcksum.py
index e838ed3..e3b1b4a 100644
--- a/gcksum.py
+++ b/gcksum.py
@@ -1,6 +1,11 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
+"""
+Calculates the Byte-checksum of a file and
+shows if it's (last Byte) correct or not.
+"""
+
 from grmn import ChkSum
 import sys
 
diff --git a/gcksum_search.py b/gcksum_search.py
new file mode 100644
index 0000000..5bbd5ba
--- /dev/null
+++ b/gcksum_search.py
@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""
+Adds up all bytes from the beginning of a file and shows where
+a byte in the file matches the expected checksum at that location.
+"""
+
+from grmn import ChkSum
+import sys
+
+FILE = sys.argv[1]
+BLOCKSIZE = 4096
+OFFSET = 0x0
+
+csum = ChkSum()
+prev_remainder = b"\x00"
+print("Reading {} ...".format(FILE))
+with open(FILE, "rb") as f:
+    while True:
+        start_pos = f.tell()
+        block = f.read(BLOCKSIZE)
+        block = block[OFFSET:]
+        for i in range(0, len(block)):
+            c = block[i:i+1]
+            exp = bytes([csum.get_expected()])
+            if c == exp:
+                print("Found matching 0x{:02x} at 0x{:x} ({:x} + {:d}).".format(c[0], OFFSET + start_pos + i, OFFSET, start_pos + i))
+            csum.add(bytes(c))
+        if len(block) < BLOCKSIZE:
+            break
+        #break
+    f.close()
diff --git a/grmn/chksum.py b/grmn/chksum.py
index 8923c70..c0de702 100644
--- a/grmn/chksum.py
+++ b/grmn/chksum.py
@@ -10,7 +10,7 @@ class ChkSum:
         self.chksum += sum(bytearray(data))
         self.last_byte = data[-1]
         self.chksum &= 0xff
-    
+
     def add_from_file(self, filename: str, print_progress: bool = False, blocksize: int=16384):
         with open(filename, "rb") as f:
             while True:
diff --git a/grmn/devices.py b/grmn/devices.py
index 3fa4974..5038f38 100644
--- a/grmn/devices.py
+++ b/grmn/devices.py
@@ -663,6 +663,7 @@ DEVICES = {
     3109: "DriveAssist 51, APAC",
     3112: "Edge 520 Plus",
     3115: "GPSMAP 64sc SiteSurvey",
+    3126: "Instinct",
     3134: "Fenix 5S Plus APAC",
     3135: "Fenix 5X Plus APAC",
     3139: "zumo 396, APAC",
diff --git a/grmn/gcd.py b/grmn/gcd.py
index 60bf275..48cf601 100644
--- a/grmn/gcd.py
+++ b/grmn/gcd.py
@@ -10,7 +10,7 @@ import sys
 GCD_SIG = b"G\x41RM\x49Nd\00"
 DEFAULT_COPYRIGHT = b"Copyright 1996-2017 by G\x61rm\x69n Ltd. or its subsidiaries."
 DEFAULT_FIRST_PADDING = 21
-DEFAULT_ALIGN = 0x1000
+DEFAULT_ALIGN = 0x1000    # second padding block pads until 0x1000
 
 TLV_TYPES = {
     0x0001: "Checksum rectifier",
@@ -28,23 +28,38 @@ TLV_TYPES = {
     0xffff: "EOF marker",
 }
 
+# Typical structure:
+# first 0x1000 Bytes: GCD_SIG > 0x0001 > 0x0002 > 0x0003 > 0x0005 > 0x0001 > 0x0002
+# then: 0x0001 > ( 0x0006 > 0x0007 > 0x???? > 0x0001 ... ) > 0xffff
+
 class TLV:
-    def __init__(self, type_id: int, expected_length: int, value=None):
+    def __init__(self, type_id: int, expected_length: int, value=None, offset: int=None):
         self.type_id = type_id
+        self.offset = offset
         self.comment = TLV_TYPES.get(type_id, "Type {:04x} / {:d}".format(type_id, type_id))
         self.length = expected_length
         self.value = None
+        self.is_parsed = False
         if value is not None:
             self.value = bytes(value)
 
     @staticmethod
-    def factory(header: bytes):
-        (type_id, length) = unpack("<HH", payload)
+    def factory(header: bytes, offset: int = None):
+        (type_id, length) = unpack("<HH", header)
         if type_id == 0x0006:
-            return TLV6(type_id, length)
+            new_tlv = TLV6(type_id, length)
         elif type_id == 0x0007:
-            return TLV7(type_id, length)
-        return TLV(type_id, length)
+            new_tlv = TLV7(type_id, length)
+        else:
+            new_tlv = TLV(type_id, length)
+        new_tlv.offset = offset
+        return new_tlv
+
+    def __str__(self):
+        plural = ""
+        if self.length != 1:
+            plural = "s"
+        return "TLV Type {:04x} at 0x{:x}, {:d} Byte{} - {}".format(self.type_id, self.offset, self.length, plural, self.comment)
 
     def set_value(self, new_value: bytes):
         self.value = new_value
@@ -60,7 +75,7 @@ class TLV:
     def get_record_length(self):
         # Length including record definition
         return self.get_actual_length() + 4
-    
+
     def get_value(self):
         return self.value
 
@@ -91,10 +106,6 @@ class TLV6(TLV):
         0x5003: ["", "End of definition marker"],
     }
 
-    def __init__(self, type_id: int, expected_length: int, value=None):
-        super.__init__(type_id, expected_length, value)
-        self.is_parsed = False
-
     def parse(self):
         if len(self.value) % 2 != 0:
             raise Exception("Invalid TLV6 payload length!")
@@ -147,8 +158,9 @@ class Gcd:
             if sig != GCD_SIG:
                 raise Exception("Signature mismatch ({}, should be {})!".format(repr(sig), repr(GCD_SIG)))
             while True:
+                cur_offset = f.tell()
                 header = f.read(4)
-                tlv = TLV.factory(header)
+                tlv = TLV.factory(header, offset=cur_offset)
                 self.struct.append(tlv)
                 if tlv.type_id == 0xFFFF:
                     # End of file reached