There are several common structures to the layout of all the firmwares
that have been examined.

For this description, the firmwares that were examined are:
*   x220 - Checked the major structures compared to x230
*   x230 - intense disassembly, with lesser checks on the rest of the
    xx30 series confirming the same details
*   x250 and x260 EC firmware - cursory checks


Encryption:
-----------

Two parts of the firmware are encrypted - the interrupt vector table at the
start of the image and area that has the encryption keys.

See zmatt's blog post[1] for more details.

[1] http://www.zmatt.net/unlocking-my-lenovo-laptop-part-3/

Outer Checksum:
---------------

The firmware has a simple whole-file checksum at 0x2fffc

Inner Checksums:
----------------

There is a table of checksum regions located at 0x2154, this table
contains pairs of start-at/stop-before addresses and is terminated with
a single 0xffffffff entry.

The firmware uses this table to checksum the whole firmware image, comparing
the results with the results table.  If there is a failed checksum, the
firmware will hang.

The results table is located at 0x2048 and is a table of 32-bit checksums.

Stable Addresses:
-----------------
These addresses have been seen to be stable in all the checked firmwares.

0x00000:        Interrupt vector table
0x001ff:        End of first encrypted region
0x00240:        EC version string
0x00268:        Copyright notice string
0x00ff0:        Start of second encrypted region
0x02048:        List of 32-bit checksum results ("inner checksum")
0x02154:        List of start/stop address pairs for checksum regions
0x02fff:        End of second encrypted region
0x2fffc:        Outer checksum

Notes:

* The x220 firmware starts the copyright string at 0x00264, which
  might just be because the default variable size on the H8S is 16bits,
  instead of the 32bit default on the ARC cpu - and that there are two
  default-sized variables before this string.

* I did not actually try to decrypt the x250 firmware - it is a different
  layout to the xx30 series, but might use the same decryption process

* The x260 was not encrypted at all, and did not have an outer checksum.
  However, the firmware file was 90112 bytes longer and the last 256 bytes
  are suspicious - perhaps they are a MAC for authenticating the firmware.