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Any ideas on what it would take to accomplish this? Like how would the extra memory be allocated without a firmware change?

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On the Pi 4, the BCM2711 supposedly supports 16 GiB (35-bit address space, with top half for peripherals and bottom half for SDRAM)[0 pg 5]. However, there seems to be some firmware limitation that limits you to 8 GiB of RAM, even if a 16 GiB chip is installed.[1] I would assume the BCM2712 would have the same address space (or bigger), and, as such, could support 16 GiB (or more) of RAM, but the firmware is written with the assumption that only 8 GiB of RAM will be installed.

The Pi firmware comes as a binary blob, but has there been any effort to reverse engineer it?

[0]: https://datasheets.raspberrypi.com/bcm2711/bcm2711-periphera...

[1]: https://www.youtube.com/watch?v=vtym0TAWNto

It's not a firmware problem, the memory controller only supports 17 bits for the rows.

So the hardware supports a memory map where 16 GiB of SDRAM could be present, but also limits itself to only 8 GiB in another area? Wow.

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