The ARM (Advanced Risc Machine) was created as a replacement for the 6502 in the BBC computer. Trial design Apple II ARM mother boards were done at Apple or by contractors but rumor has it that they outperformed the new Macintosh and were scraped. Here is an ARM 7500 board designed by a friend of mine. It takes all PC peripherals like mouse, keyboard, video, and IDE drives, and has a single PC slot. It is based on a VLSI part and is a 32 bit RISC (Reduced Instruction Set Computer) that in this case runs at 40 MHz if memory serves.
I have started ARM based projects several times over the last 20 years. On the left are an ARM and a "glue" chip for a full 32 bit data buss system. They are on breakout boards to allow prototyping in a wire-wrapped setup. On the right is a development board with a Phillips ARM 2106 Thumb processor with nearly everything internal. Yes, it is the little square black thing and has 64 K of RAM and 256 K of Flash built in. Notice that the only glue logic is an RS232 converter chip near the connector. The other stuff is just a crystal and two voltage regulators for the 1.8v and 3.3v needed for the 60 MHz chip.
Here is the same stuff plus a Sharp ARM at the bottom in the quad flat pack and two Atmel ARMs in BGA (Ball Grid Array - one is upside down so you can see the solder balls) packages that allow more external memory or devices than the Phillips part in the board on the right. One of these days I'm going to finish an ARM board that fits an Apple IIe case and uses the Apple keyboard. Why? I don't know.
Now we come to a 32 bit orphan. National Semiconductor began a line of very interesting processors and some DEC compatible boards. DEC put a stop through legal action, but the processors were not affected. The top of the line was a 32 bit chip well ahead of its time in some ways. If I can recall without looking it up, the registers were in RAM so context switching was easy as there were always more registers. You just moved the base pointer. In today's speed environment this would all have to be in very high speed cache or on-chip storage and I think the advantage is lost. However, I wlked into Fry's in Sunnyvale one afternoon and there was a stack of these NS 32000 designer kits for less than $100. It had a heavy coated carboard box and a plastic cary handle! So I bought one thinking I would wire wrap a little system to play with. I may do it yet.
The box had the 32 bit processor in a kind of 68 pin PLCC and it took ages to find the proto board you see it snapped into here. Also included was the ROM monitor and some glue chips with the data book and underneath is a big instruction set document for a three ring binder.
Here are the components. Some day I must put it together and write a Forth compiler.