micropython: add micropython component

components/language/micropython/docs/develop/memorymgt.rst

@@ -0,0 +1,141 @@
+.. _memorymanagement:
+
+Memory Management
+=================
+
+Unlike programming languages such as C/C++, MicroPython hides memory management
+details from the developer by supporting automatic memory management.
+Automatic memory management is a technique used by operating systems or applications to automatically manage
+the allocation and deallocation of memory. This eliminates challenges such as forgetting to
+free the memory allocated to an object. Automatic memory management also avoids the critical issue of using memory
+that is already released. Automatic memory management takes many forms, one of them being
+garbage collection (GC).
+
+The garbage collector usually has two responsibilities;
+
+#. Allocate new objects in available memory.
+#. Free unused memory.
+
+There are many GC algorithms but MicroPython uses the
+`Mark and Sweep <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Basic_algorithm>`_
+policy for managing memory. This algorithm has a mark phase that traverses the heap marking all
+live objects while the sweep phase goes through the heap reclaiming all unmarked objects.
+
+Garbage collection functionality in MicroPython is available through the ``gc`` built-in
+module:
+
+.. code-block:: bash
+
+   >>> x = 5
+   >>> x
+   5
+   >>> import gc
+   >>> gc.enable()
+   >>> gc.mem_alloc()
+   1312
+   >>> gc.mem_free()
+   2071392
+   >>> gc.collect()
+   19
+   >>> gc.disable()
+   >>>
+
+Even when ``gc.disable()`` is invoked, collection can be triggered with ``gc.collect()``.
+
+The object model
+----------------
+
+All MicroPython objects are referred to by the ``mp_obj_t`` data type.
+This is usually word-sized (i.e. the same size as a pointer on the target architecture),
+and can be typically 32-bit (STM32, nRF, ESP32, Unix x86) or 64-bit (Unix x64).
+It can also be greater than a word-size for certain object representations, for
+example ``OBJ_REPR_D`` has a 64-bit sized ``mp_obj_t`` on a 32-bit architecture.
+
+An ``mp_obj_t`` represents a MicroPython object, for example an integer, float, type, dict or
+class instance. Some objects, like booleans and small integers, have their value stored directly
+in the ``mp_obj_t`` value and do not require additional memory. Other objects have their value
+store elsewhere in memory (for example on the garbage-collected heap) and their ``mp_obj_t`` contains
+a pointer to that memory. A portion of ``mp_obj_t`` is the tag which tells what type of object it is.
+
+See ``py/mpconfig.h`` for the specific details of the available representations.
+
+**Pointer tagging**
+
+Because pointers are word-aligned, when they are stored in an ``mp_obj_t`` the
+lower bits of this object handle will be zero.  For example on a 32-bit architecture
+the lower 2 bits will be zero:
+
+``********|********|********|******00``
+
+These bits are reserved for purposes of storing a tag. The tag stores extra information as
+opposed to introducing a new field to store that information in the object, which may be
+inefficient.  In MicroPython the tag tells if we are dealing with a small integer, interned
+(small) string or a concrete object, and different semantics apply to each of these.
+
+For small integers the mapping is this:
+
+``********|********|********|*******1``
+
+Where the asterisks hold the actual integer value.  For an interned string or an immediate
+object (e.g. ``True``) the layout of the ``mp_obj_t`` value is, respectively:
+
+``********|********|********|*****010``
+
+``********|********|********|*****110``
+
+While a concrete object that is none of the above takes the form:
+
+``********|********|********|******00``
+
+The stars here correspond to the address of the concrete object in memory.
+
+Allocation of objects
+----------------------
+
+The value of a small integer is stored directly in the ``mp_obj_t`` and will be
+allocated in-place, not on the heap or elsewhere.  As such, creation of small
+integers does not affect the heap.  Similarly for interned strings that already have
+their textual data stored elsewhere, and immediate values like ``None``, ``False``
+and ``True``.
+
+Everything else which is a concrete object is allocated on the heap and its object structure is such that
+a field is reserved in the object header to store the type of the object.
+
+.. code-block:: bash
+
+    +++++++++++
+    +         +
+    + type    + object header
+    +         +
+    +++++++++++
+    +         + object items
+    +         +
+    +         +
+    +++++++++++
+
+The heap's smallest unit of allocation is a block, which is four machine words in
+size (16 bytes on a 32-bit machine, 32 bytes on a 64-bit machine).
+Another structure also allocated on the heap tracks the allocation of
+objects in each block. This structure is called a *bitmap*.
+
+.. image:: img/bitmap.png
+
+The bitmap tracks whether a block is "free" or "in use" and use two bits to track this state
+for each block.
+
+The mark-sweep garbage collector manages the objects allocated on the heap, and also
+utilises the bitmap to mark objects that are still in use.
+See `py/gc.c <https://github.com/micropython/micropython/blob/master/py/gc.c>`_
+for the full implementation of these details.
+
+**Allocation: heap layout**
+
+The heap is arranged such that it consists of blocks in pools. A block
+can have different properties:
+
+- *ATB(allocation table byte):* If set, then the block is a normal block
+- *FREE:* Free block
+- *HEAD:* Head of a chain of blocks
+- *TAIL:* In the tail of a chain of blocks
+- *MARK :* Marked head block
+- *FTB(finaliser table byte):* If set, then the block has a finaliser