Theo  wrote:
> Areligious Republican  wrote:
> > Is the memory bus visible at all to allow the addition
> > of (unlimited) I/O expansion?
>
> No.

It turns out there is a 'Secondary Memory Interface' (SMI) which can be used
to talk to slave devices, eg:
https://github.com/fenlogic/IDE_trial
https://www.raspberrypi.org/forums/viewtopic.php?t=92233#p1008403
ALT1 here:
https://elinux.org/RPi_BCM2835_GPIOs
but there are only 6 bits of address, so you can't address very large memory
spaces without banking.

Theo

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