42 lines
2 KiB
ReStructuredText
42 lines
2 KiB
ReStructuredText
Memory allocation
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=================
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Overview
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--------
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The ESP32 has multiple types of RAM. Internally, there's IRAM, DRAM as well as RAM that can be used as both. It's also
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possible to connect external SPI flash to the ESP32; it's memory can be integrated into the ESP32s memory map using
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the flash cache.
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In order to make use of all this memory, esp-idf has a capabilities-based memory allocator. Basically, if you want to have
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memory with certain properties (for example, DMA-capable, accessible by a certain PID, or capable of executing code), you
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can create an OR-mask of the required capabilities and pass that to pvPortMallocCaps. For instance, the normal malloc
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code internally allocates memory with ```pvPortMallocCaps(size, MALLOC_CAP_8BIT)``` in order to get data memory that is
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byte-addressable.
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Because malloc uses this allocation system as well, memory allocated using pvPortMallocCaps can be freed by calling
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the standard ```free()``` function.
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Internally, this allocator is split in two pieces. The allocator in the FreeRTOS directory can allocate memory from
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tagged regions: a tag is an integer value and every region of free memory has one of these tags. The esp32-specific
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code initializes these regions with specific tags, and contains the logic to select applicable tags from the
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capabilities given by the user. While shown in the public API, tags are used in the communication between the two parts
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and should not be used directly.
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Special Uses
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------------
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If a certain memory structure is only addressed in 32-bit units, for example an array of ints or pointers, it can be
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useful to allocate it with the MALLOC_CAP_32BIT flag. This also allows the allocator to give out IRAM memory; something
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which it can't do for a normal malloc() call. This can help to use all the available memory in the ESP32.
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API Reference - Heap Allocation
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-------------------------------
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.. include:: /_build/inc/esp_heap_alloc_caps.inc
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API Reference - Heap Regions
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----------------------------
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.. include:: /_build/inc/heap_regions.inc
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