Memory map limitation affecting rBoot

I’ve become aware of a serious limitation that testing should should have found, if I’d done a bit more of it! The ESP8266 only memory maps the the first 8 Mbit of the SPI flash. rBoot doesn’t use memory mapped flash, it uses SPI reads, so rBoot itself is fine with any size of flash. The problem comes if you try to put an irom section beyond 8 MBit (i.e. from address 0x40300000) – code there won’t be accessible at run time!

I’m quite disappointed about that. It’s something that rBoot can’t overcome, it’s a limitation of the ESP8266 design. You can still use the extra space on larger chips for something, like logging data, or for storing a filesystem for your resources. You can also still use rBoot OTA to flash these resources by dedicating a non-bootable rom slot to them. Your code would need to access them with SPI reads rather than through the memory mapping though.

Update: this problem has been largely mitigated, see here.

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