forked from Archive/PX4-Autopilot
93 lines
3.8 KiB
Plaintext
93 lines
3.8 KiB
Plaintext
mm/README.txt
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=============
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This directory contains the NuttX memory management logic. This include:
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1) The standard memory management functions as prototyped in stdlib.h as
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specified in the Base definitions volume of IEEE Std 1003.1-2001. This
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include the files:
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o Standard Interfaces: mm_malloc.c, mm_calloc.c, mm_realloc.c,
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mm_memalign.c, mm_free.c
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o Less-Standard Interfaces: mm_zalloc.c, mm_mallinfo.c
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o Internal Implementation: mm_initialize.c mm_sem.c mm_addfreechunk.c
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mm_size2ndx.c mm_shrinkchunk.c, mm_internal.h
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o Build and Configuration files: Kconfig, Makefile
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Memory Models:
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o Small Memory Model. If the MCU supports only 16-bit data addressing
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then the small memory model is automatically used. The maximum size
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of the heap is then 64K. The small memory model can also be forced
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MCUs with wider addressing by defining CONFIG_SMALL_MEMORY in the
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NuttX configuration file.
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o Large Memory Model. Otherwise, the allocator uses a model that
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supports a heap of up to 4G.
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This implementation uses a variable length allocator with the following
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properties:
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o Overhead: Either 8- or 4-bytes per allocation for large and small
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models, respectively.
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o Alignment: All allocations are aligned to 8- or 4-bytes for large
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and small models, respectively.
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2) Test Program. There is also a host-best test program that can be
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used to verify the memory manager. These are the file:
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Makefile.test, mm_test.c, and mm_environment.h.
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Build instructions:
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make -f Makefile.test
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The executable will be built in the top-level directory as nuttx/mm_text
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(or mm_test.exe under Cygwin).
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3) Granule Allocator. A non-standard granule allocator is also available
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in this directory The granule allocator allocates memory in units
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of a fixed sized block ("granule"). Allocations may be aligned to a user-
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provided address boundary.
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The granule allocator interfaces are defined in nuttx/include/nuttx/gran.h.
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The granule allocator consists of these files in this directory:
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mm_gran.h, mm_granalloc.c, mm_grancritical.c, mm_granfree.c
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mm_graninit.c
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The granule allocator is not used anywhere within the base NuttX code
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as of this writing. The intent of the granule allocator is to provide
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a tool to support platform-specific management of aligned DMA memory.
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NOTE: Because each granule may be aligned and each allocation is in
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units of the granule size, selection of the granule size is important:
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Larger granules will give better performance and less overhead but more
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losses of memory due to quantization waste. Additional memory waste
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can occur from alignment; Of course, heap alignment should no be
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used unless (a) you are using the granule allocator to manage DMA memory
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and (b) your hardware has specific memory alignment requirements.
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The current implementation also restricts the maximum allocation size
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to 32 granules. That restriction could be eliminated with some
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additional coding effort, but currently requires larger granule
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sizes for larger allocations.
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Geneneral Usage Example. This is an example using the GCC section
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attribute to position a DMA heap in memory (logic in the linker script
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would assign the section .dmaheap to the DMA memory.
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FAR uint32_t g_dmaheap[DMAHEAP_SIZE] __attribute__((section(.dmaheap)));
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The heap is created by calling gran_initialize. Here the granual size
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is set to 64 bytes and the alignment to 16 bytes:
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GRAN_HANDLE handle = gran_initialize(g_dmaheap, DMAHEAP_SIZE, 6, 4);
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Then the GRAN_HANDLE can be used to allocate memory (There is no
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GRAN_HANDLE if CONFIG_GRAN_SINGLE=y):
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FAR uint8_t *dma_memory = (FAR uint8_t *)gran_alloc(handle, 47);
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The actual memory allocates will be 64 byte (wasting 17 bytes) and
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will be aligned at least to (1 << log2align).
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