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mm/README.txt
=============
This directory contains the NuttX memory management logic. This include:
1) The standard memory management functions as prototyped in stdlib.h as
specified in the Base definitions volume of IEEE Std 1003.1-2001. This
include the files:
o Standard Interfaces: mm_malloc.c, mm_calloc.c, mm_realloc.c,
mm_memalign.c, mm_free.c
o Less-Standard Interfaces: mm_zalloc.c, mm_mallinfo.c
o Internal Implementation: mm_initialize.c mm_sem.c mm_addfreechunk.c
mm_size2ndx.c mm_shrinkchunk.c, mm_internal.h
o Build and Configuration files: Kconfig, Makefile
Memory Models:
o Small Memory Model. If the MCU supports only 16-bit data addressing
then the small memory model is automatically used. The maximum size
of the heap is then 64K. The small memory model can also be forced
MCUs with wider addressing by defining CONFIG_SMALL_MEMORY in the
NuttX configuration file.
o Large Memory Model. Otherwise, the allocator uses a model that
supports a heap of up to 4G.
This implementation uses a variable length allocator with the following
properties:
o Overhead: Either 8- or 4-bytes per allocation for large and small
models, respectively.
o Alignment: All allocations are aligned to 8- or 4-bytes for large
and small models, respectively.
2) Test Program. There is also a host-best test program that can be
used to verify the memory manager. These are the file:
Makefile.test, mm_test.c, and mm_environment.h.
Build instructions:
make -f Makefile.test
The executable will be built in the top-level directory as nuttx/mm_text
(or mm_test.exe under Cygwin).
3) Granule Allocator. A non-standard granule allocator is also available
in this directory The granule allocator allocates memory in units
of a fixed sized block ("granule"). All memory is aligned to the size
of one granule.
The granule allocator interfaces are defined in nuttx/include/nuttx/gran.h.
The granule allocator consists of these files in this directory:
mm_gran.h, mm_granalloc.c, mm_grancritical.c, mm_granfree.c
mm_graninit.c
The granule allocator is not used anywhere within the base NuttX code
as of this writing. The intent of the granule allocator is to provide
a tool to support platform-specific management of aligned DMA memory.
NOTE: Because each granule is aligned to the granule size and each
allocations is in units of the granule size, selection of the granule
size is important: Larger granules will give better performance and
less overhead but more losses of memory due to alignment and quantization
waste.
The current implementation also restricts the maximum allocation size
to 32 granules. That restriction could be eliminated with some
additional coding effort, but currently requires larger granule
sizes for larger allocations.
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