Coco/Atari OS9 new hardware

[commodorejohn]

The CDi is good for exactly one thing, and that's the hilariously God-awful Nintendo-licensed games, and the best parts of those are all over YouTube anyway.

[commodorejohn]

The thing I've always been amazed by is the fact that an 8 bit OS could support a real time multi-tasking, multi-user operating system.

I've seen a Color Computer running this driving three VT100 terminals and operating a Point of Sale system.

I don't think it's all that much of a shocker; people think preemptive multi-tasking and multi-user capability are more advanced than they really are because it took so long for Mac and Windows machines to get them, but even by the late '60s mainframes could run OSes implementing these features. The 6809 (like the 6502) also has the advantage that there aren't so many registers to save as modern CPUs have, so context switches are fairly quick.

[commodorejohn]

However, multi-tasking on early Intel processor was difficult, operated poorly, and wasn't very useful. Until the i386 came out, it didn't really make sense.

Blame the segment-register kluge - without an MMU, programs are either stuck with 64KB code + 64KB data, or they play around with the segment registers themselves and imperil system stability, or the OS has to do any segment-switching for the program, which would be cumbersome as all hell. (The 286 had an MMU, but bone-headedly it wasn't compatible with ordinary 8086 code such as any given PC BIOS and went essentially unused - I think Xenix might've supported it, but that's it.) The 386 was the first to combine the ideas that "memory-mapping would be useful" and "but it should also work with the software and hardware everybody's already using" and come up with something useful.

[commodorejohn]

Yup, it does sound like a good design for a small-memory system.

Nothing like space constraints to bring out the best in programmers...

[commodorejohn]

In some ways a 16 bit 6502C replacement would make more sense as the 6809 isn't 6502 compatible (so no existing software is going to run).

It would, except the 65C816 is kind of a compromised design. It's good for providing expanded capability, higher clock speeds, and near-perfect backwards compatibility, but it's nothing you'd really want to build an OS on top of.

[commodorejohn]

WDC's old site is still up, so you can get a lot of information, including the datasheet, from their 65816 page. If you're just looking for a quick reference to the architecture, the SNES homebrew community has some good guides; here's a pretty comprehensive one.