This example uses the [Himem API](https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/system/himem.html) to run a memory test of the upper 4 MiB of an 8 MiB PSRAM chip.
The ESP32 has the ability to access external SPI RAM in the same way internal memory can be accessed. In other words, if enabled in project configuration, you can allocate memory in external RAM using standard C allocation APIs like `malloc()`, `calloc()`, etc.
However, because of hardware limitations, this only works for up to 4 MiB of external memory. If you have, for instance, an ESP32-WROVER module with 8 MiB of PSRAM, you cannot use the upper 4 MiB of PSRAM this way. However, this memory is not wasted, using the Himem API (which essentially is a bank switching scheme for the upper memory regions), the upper 4 MiB is still usable.
The Himem subsystem does this by reserving some amount of address space, then allowing applications to swap in and out normally unreachable ranges of physical SPI RAM. While this does not allow transparent access in the way memory allocated with `malloc()` does, it does provide an usable way to store data for e.g., large audio or video buffers in the upper 4 MiB.
This example uses the Himem API to run a simple memory test of the entire range of upper memory. It illustrates how to allocate address space to bankswitch the physical memory in, allocate the physical memory, and switch it in or out of the allocated address space.
This example requires an ESP32 with external SPI RAM connected, for instance an ESP32-WROVER module. The example is intended to run on an ESP32 with 8 MiB external RAM connected. It will still run on an ESP32 with 4 MiB external RAM, but in practice using Himem with such a setup does not make much sense.
* Make sure that SPI RAM bank switching is enabled. (Compiling the example with default values will automatically enable this.) It can be found under `Component config > ESP32-specific > Support for external, SPI-connected RAM > SPI RAM config`.
