{IDF_TARGET_NAME} Memory Management Unit (MMU) is relatively simple. It can do memory address translation between physical memory addresses and virtual memory addresses. So CPU can access physical memories via virtual addresses. There are multiple types of virtual memory addresses, which have different capabilities.
ESP-IDF provides a memory mapping driver that manages the relation between these physical memory addresses and virtual memory addresses, so as to achieve some features such as reading from SPI Flash via a pointer.
Memory mapping driver is actually a capabilities-based virtual memory address allocator that allows apps to make virtual memory address allocations for different purposes. In the following chapters, we call this driver ``esp_mmap`` driver.
ESP-IDF also provides a memory synchronisation driver which can be used for potential memory desychronisation scenarios.
Physical Memory Types
=====================
Memory mapping driver currently supports mapping to following physical memory types:
..list::
- SPI Flash
:SOC_SPIRAM_SUPPORTED and not esp32:- PSRAM
Virtual Memory Capabilities
===========================
..list::
-:cpp:enumerator:`MMU_MEM_CAP_EXEC`. This capability indicates that the virtual memory address has the execute permission. Note this permission scope is within the MMU hardware.
-:cpp:enumerator:`MMU_MEM_CAP_READ`. This capability indicates that the virtual memory address has the read permission. Note this permission scope is within the MMU hardware.
-:cpp:enumerator:`MMU_MEM_CAP_WRITE`. This capability indicates that the virtual memory address has the write permission. Note this permission scope is within the MMU hardware.
-:cpp:enumerator:`MMU_MEM_CAP_32BIT`. This capability indicates that the virtual memory address allows for 32 bits or multiples of 32 bits access.
-:cpp:enumerator:`MMU_MEM_CAP_8BIT`. This capability indicates that the virtual memory address allows for 8 bits or multiples of 8 bits access.
..only:: esp32
8 MB external memory addresses (from 0x40400000 to 0x40C00000) which have the :cpp:enumerator:`MMU_MEM_CAP_EXEC` and :cpp:enumerator:`MMU_MEM_CAP_READ` capabilities are not avaiable for users to allocate, due to hardware limitations.
..only:: esp32s2
4 MB external memory addresses (from 0x40400000 to 0x40800000) which have the :cpp:enumerator:`MMU_MEM_CAP_EXEC` and :cpp:enumerator:`MMU_MEM_CAP_READ` capabilities are not avaiable for users to allocate, due to hardware limitations.
You can call :cpp:func:`esp_mmu_map_get_max_consecutive_free_block_size` to know the largest consecutive mappable block size with certain capabilities.
Memory Management Drivers
=========================
Driver Concept
--------------
Terminology
^^^^^^^^^^^
The virtual memory pool is made up with one or multiple virtual memory regions, see below figure:
..image:: /../_static/diagrams/mmu/mem_pool.png
:scale:100 %
:align:center
- A virtual memory pool stands for the whole virtual address range that can be mapped to physical memory
- A virtual memory region is a range of virtual address with same attributes
- A virtual memory block is a piece of virtual address range that is dynamically mapped.
- A slot is the virtual address range between two virtual memory blocks.
- A physical memory block is a piece of physical address range that is to-be-mapped or already mapped to a virtual memory block.
- Dynamical mapping is done by calling ``esp_mmap`` driver API :cpp:func:`esp_mmu_map`, this API will map the given physical memory block to a virtual memory block which is allocated by the ``esp_mmap`` driver.
You can call :cpp:func:`esp_mmu_map` to do a dynamical mapping. This API can allocate a certain size of virtual memory block according to the virtual memory capabilities you selected, then map this virtual memory block to the physical memory block as you requested. The ``esp_mmap`` driver supports mapping to one or more types of physical memory, so you should specify the physical memory target when mapping.
* If it is the enclosed scenario, this API will return an :c:macro:`ESP_ERR_INVALID_STATE`. The ``out_ptr`` will be assigned to the start virtual memory address of the previously mapped one which encloses the to-be-mapped one.
* If it is the identical scenario, this API will behaves exactly the same as the enclosed scenario.
* If it is the overlapped scenario, this API will by default return an :c:macro:`ESP_ERR_INVALID_ARG`. This means, ``esp_mmap`` driver by default does not allow mapping a physical memory address to multiple virtual memory addresses.
Specially, you can use :c:macro:`ESP_MMU_MMAP_FLAG_PADDR_SHARED`. This flags stands for one-to-multiple mapping between a physical address and multiple virtual addresses:
You can call :cpp:func:`esp_mmu_unmap` to unmap a previously mapped memory block. This API returns an :c:macro:`ESP_ERR_NOT_FOUND` if you are trying to unmap a virtual memory block that is not mapped to any physical memory block yet.
SPI Flash can be accessed by SPI1 (ESP-IDF ``esp_flash`` driver APIs), or by pointers. ESP-IDF ``esp_flash`` driver APIs have already considered the memory synchronisation, so users do not need to worry about this.
PSRAM can be accessed by pointers, hardware guarantees the data consistency when PSRAM is only accessed via pointers.
..only:: esp32s3
PSRAM can also be accessed by EDMA. Data desynchronisation may happen because hardware does not guarantee the data consistency under such condition. You should call :cpp:func:`esp_cache_msync` to synchronise the Cache and the PSRAM.
