Running Rust code on the Raspberry Pi Pico

The RP2040 has external QSPI flash. There is an internal mask-ROM bootloader which can read the first 256 bytes of external flash and copy it to the top of SRAM (0x2004_1f00). This 256 byte block is the 'second stage bootloader'. Its job is to reconfigure the XIP_SSI interface to the right values for the attached QSPI flash chip (the mask ROM has some very conservative defaults which should work with all chips), and configure the XIP so that the QSPI flash appears at address 0x1000_0000 upwards.

The second stage bootloader then loads the user application once the XIP_SSI configuration is complete. It does this by reading a vector table at 0x1000_0100, which is 256 bytes into the QSPI flash contents (or immediately after the 256 byte second stage bootloader), and jumping to the reset vector.

We should also write a BSP for the pico, and a HAL for the RP2040. Basically, don't do any of what I did here. But hey, it blinks!

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