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34 lines
1.7 KiB
ReStructuredText
34 lines
1.7 KiB
ReStructuredText
The himem allocation API
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========================
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Overview
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--------
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The ESP32 can access external SPI RAM transparently, so you can use it as normal memory in your program code. However, because the address
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space for external memory is limited in size, only the first 4MiB can be used as such. Access to the remaining memory is still possible,
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however this needs to go through a bankswitching scheme controlled by the himem API.
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Specifically, what is implemented by the himem API is a bankswitching scheme. Hardware-wise, the 4MiB region for external SPI RAM is
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mapped into the CPU address space by a MMU, which maps a configurable 32K bank/page of external SPI RAM into each of the 32K pages in the
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4MiB region accessed by the CPU. For external memories that are <=4MiB, this MMU is configured to unity mapping, effectively mapping each
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CPU address 1-to-1 to the external SPI RAM address.
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In order to use the himem API, you have to enable it in the menuconfig using :envvar:`CONFIG_SPIRAM_BANKSWITCH_ENABLE`, as well as set the amount
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of banks reserved for this in :envvar:`CONFIG_SPIRAM_BANKSWITCH_RESERVE`. This decreases
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the amount of external memory allocated by functions like ``malloc()``, but it allows you to use the himem api to map any of the remaining memory
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into the reserved banks.
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The himem API is more-or-less an abstraction of the bankswitching scheme: it allows you to claim one or more banks of address space
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(called 'regions' in the API) as well as one or more of banks of memory to map into the ranges.
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Example
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-------
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An example doing a simple memory test of the high memory range is available in esp-idf: :example:`system/himem`
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API Reference
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-------------
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.. include-build-file:: inc/himem.inc
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