I'd love to be able to buy and ARM cpu card, mostly a RPi with a pci car form factor, to plug into my main computer and run android programs without emulation.
Considering development benefits of such a system, specially more complete ones with video output, why isn't it used these days?
At the low end of complexity, you could put a raspberry pi on a pcie daughterboard and just power it off the slot, and maybe fake a NIC to the host for file transfer.
That's not really any more useful than just having a separate, dedicated raspberry pi though. To be useful, you need to implement some sort of deeper synchronization. Perhaps you could have a shared memory aperture between the two systems, a virtio style interface (but physical, not virtual!) with the associated protocols for passing through files, visuals, etc.
But it's all complicated and expensive, and in the end unless you're doing CPU level debugging and need to sync a debugger with low level CPU state, there's really not much advantage besides some space saving to just having a separate x/y/z on your desk.
https://en.wikipedia.org/wiki/Z-80_SoftCard
https://en.wikipedia.org/wiki/Apple_IIe_Card
https://www.applerepairmanuals.com/the_manuals_are_in_here/P...
Many Apple IIs in the business world were only used to run CPM.
see Wikipedia: https://en.wikipedia.org/wiki/Z-80_SoftCard
"The SoftCard was the single most-popular platform to run CP/M"
One of the more interesting products we resold were the ROSS Technology "Sparcplugs" - you basically had a SPARC motherboard and CPU that fit into a couple of 5.25" drive bays on a host PC. So almost the reverse of the SunPCI cards.