I wonder if it would be possible to use a widely available PC ISA motherboard as base for a 65xx-system by taking over bus control from the host CPU:

Source: hardwarebook

If that should be possible one could even use system memory and of course all ISA hw out there. (Don't focus on address space - there might be a solution for that. Simple but very slow: attach ISA not directly to CPU but via I/O ics)

Designing a bus mastering ISA card is harder than just designing an ISA card that decodes a couple of I/O addresses to communicate. The only advantage of bus mastering is that you can access the main board memory from your card.

But let's say you would want to access a hard disk or network card or video card from your 6502 card. You would have to jump through all kinds of loops to take control of the bus, but not too much or you could corrupt the main board RAM. Besides there's also DMA going on for disk access and other fun stuff. And if you would get that right, you'd have to write and debug a lot of software (in 6502 assembly) to make all that hardware work for you.

I would probably let the 80x86 processor and the BIOS ROM on the motherboard take care of the heavy lifting. I would write a program that runs on the 80x86 and polls for requests from the 6502 and executes them according to some self-made protocol. You can make the mainboard CPU load and run that program from a floppy or hard disk under DOS, or you can store it on a boot sector of a floppy disk, or you can put it in a ROM that you put on your board, so that the BIOS will execute it automatically. As soon as the 80x86 "makes contact" with the 6502 on your board, the 6502 can tell it to print characters to the screen with BIOS Int 10h, read and write floppy drive or hard drive sectors with Int 13h, read the keyboard with Int 16h etc.

===Jac