docs/reference/mpyfiles: Update .mpy description to match latest format.

Signed-off-by: Damien George <damien@micropython.org>

docs/reference/mpyfiles.rst

@@ -80,7 +80,8 @@ and .mpy version.
 =================== ============
 MicroPython release .mpy version
 =================== ============
-v1.12 and up        5
+v1.19 and up        6
+v1.12 - v1.18       5
 v1.11               4
 v1.9.3 - v1.10      3
 v1.9 - v1.9.2       2
@@ -93,6 +94,7 @@ MicroPython repository at which the .mpy version was changed.
 =================== ========================================
 .mpy version change Git commit
 =================== ========================================
+5 to 6              f2040bfc7ee033e48acef9f289790f3b4e6b74e5
 4 to 5              5716c5cf65e9b2cb46c2906f40302401bdd27517
 3 to 4              9a5f92ea72754c01cc03e5efcdfe94021120531e
 2 to 3              ff93fd4f50321c6190e1659b19e64fef3045a484
@@ -104,21 +106,31 @@ initial version 0   d8c834c95d506db979ec871417de90b7951edc30
 Binary encoding of .mpy files
 -----------------------------
 
-MicroPython .mpy files are a binary container format with code objects
-stored internally in a nested hierarchy.  To keep files small while still
+MicroPython .mpy files are a binary container format with code objects (bytecode
+and native machine code) stored internally in a nested hierarchy.  The code for
+the outer module is stored first, and then its children follow.  Each child may
+have further children, for example in the case of a class having methods, or a
+function defining a lambda or comprehension.  To keep files small while still
 providing a large range of possible values it uses the concept of a
 variably-encoded-unsigned-integer (vuint) in many places.  Similar to utf-8
 encoding, this encoding stores 7 bits per byte with the 8th bit (MSB) set
 if one or more bytes follow.  The bits of the unsigned integer are stored
 in the vuint in LSB form.
 
-The top-level of an .mpy file consists of two parts:
+The top-level of an .mpy file consists of three parts:
 
 * The header.
 
+* The global qstr and constant tables.
+
 * The raw-code for the outer scope of the module.
   This outer scope is executed when the .mpy file is imported.
 
+You can inspect the contents of a .mpy file by using ``mpy-tool.py``, for
+example (run from the root of the main MicroPython repository)::
+
+    $ ./tools/mpy-tool.py -xd myfile.mpy
+
 The header
 ~~~~~~~~~~
 
@@ -131,7 +143,26 @@ byte    value 0x4d (ASCII 'M')
 byte    .mpy version number
 byte    feature flags
 byte    number of bits in a small int
-vuint   size of qstr window
+======  ================================
+
+The global qstr and constant tables
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An .mpy file contains a single qstr table, and a single constant object table.
+These are global to the .mpy file, they are referenced by all nested raw-code
+objects.  The qstr table maps internal qstr number (internal to the .mpy file)
+to the resolved qstr number of the runtime that the .mpy file is imported into.
+This links the .mpy file with the rest of the system that it executes within.
+The constant object table is populated with references to all constant objects
+that the .mpy file needs.
+
+======  ================================
+size    field
+======  ================================
+vuint   number of qstrs
+vuint   number of constant objects
+...     qstr data
+...     encoded constant objects
 ======  ================================
 
 Raw code elements
@@ -143,24 +174,21 @@ contents are:
 ======  ================================
 size    field
 ======  ================================
-vuint   type and size
+vuint   type, size and whether there are sub-raw-code elements
 ...     code (bytecode or machine code)
-vuint   number of constant objects
-vuint   number of sub-raw-code elements
-...     constant objects
+vuint   number of sub-raw-code elements (only if non-zero)
 ...     sub-raw-code elements
 ======  ================================
 
 The first vuint in a raw-code element encodes the type of code stored in this
-element (the two least-significant bits), and the decompressed length of the code
-(the amount of RAM to allocate for it).
-
-Following the vuint comes the code itself.  In the case of bytecode it also contains
-compressed qstr values.
+element (the two least-significant bits), whether this raw-code has any
+children (the third least-significant bit), and the length of the code that
+follows (the amount of RAM to allocate for it).
 
-Following the code comes a vuint counting the number of constant objects, and
-another vuint counting the number of sub-raw-code elements.
+Following the vuint comes the code itself.  Unless the code type is viper code
+with relocations, this code is constant data and does not need to be modified.
 
-The constant objects are then stored next.
+If this raw-code has any children (as indicated by a bit in the first vuint),
+following the code comes a vuint counting the number of sub-raw-code elements.
 
 Finally any sub-raw-code elements are stored, recursively.