For $595, you get what nobody else can give you for twice the price (1982) [pdf]

Those reference cards are very helpful. In college the embedded PC boards (generic) x86 and (I forget if it was a 68 or 65 series CPU) each included a CPU manual and quick assembly reference card. Seeing instructions grouped by how the decoder wanted them, it was much easier to see where a given entry would have 3 or 4 bits utilized to decode a sub function among a general class of instructions.

Sadly I couldn't find a good link to a quick reference card, but there are some copies of the CPU manual on https://archive.org/search?query=6809 . A Wikipedia site is nowhere near as good as a properly typeset and slightly grouped to convey clarity card, but it's still useful https://github.com/Ta0uf19/Motorola-6809-Cheatsheet

> Sounds like a great little machine, actually!

Yes, it was quite decent and attracted a strong community of advanced hobbyists who were attracted by the 6809 CPU's power. The Coco had quite a lot of third-party hardware upgrades made by hobbyist garage companies and by leveraging these it could be inexpensively upgraded to quite a nice system, much like the Apple II's expansion capability - except the Coco had no internal slots so we had to solder wires to chip pins (another good learning lesson :-)).

Radio Shack wasn't really a full computer company like Atari, Commodore, Apple, Sinclair, etc. They were primarily an electronics retailer who did their own manufacturing to drive down costs but only dabbled in original computer design. While they did a few somewhat unique designs like the Model 1, 2, 3, 4 series of 6502 machines, they tended to stick close to designs based on off-the-shelf chip manufacturer parts and didn't often do custom chips. Some of their computers were even private label rebrands of other manufacturer's computers, like the Model 100/200 and all the Pocket Computers.

Because of this, Radio Shack's Color Computer was a straight off-the-shelf implementation of Motorola's 6809 system reference design - and Motorola had no cool graphics hardware support in their 6847 video chip. However, the upside of no GFX hardware was all that money went to the CPU which was two to three times more expensive than the 6502 or Z80. In fact, Woz wanted to use the 6809's earlier ancestor the 6800 in the Apple I but they were far too expensive at the time. Many don't know that most of the team that created the 6502 actually worked at Motorola designing the 6800 but were frustrated that when visiting customers all they heard was "Great chip, way too expensive" so they left and founded MOSTEK to create a cheaper, less powerful 6800 clone. Their first CPU, the 6501, was actually compatible with the 6800 but Motorola sued them so they changed it enough to be incompatible and called it the 6502. Due in part to the lawsuit sapping their resources, MOSTEK started running out of money before completing the 6502 and that's how Commodore was able to buy their own chip company at a good discount.

The UK had the very similar Dragon 32 and 64 computers (and I have both!). They had the same hardware design but included a hardware serial port chip instead of a software bit-banger port. Software was mostly compatible between the Coco and Dragons as they used the same ROM-based Extended Color BASIC licensed from Microsoft. Dragons were also sold in Spain for a while and very briefly in the US by another company Dragon licensed. I think there was also a Coco-alike in Australia but can't recall the name. I don't think of these systems as Coco clones because they were all based on the same Motorola reference design and Microsoft ROM BASIC, which Motorola created to drive sales of their 6809 chip family (6883 system controller, 6847 display chip and 6821 UART).

The CIA 6526 shift register on the C64 is not used. Because the trace on the pcb that connected to it was accidentally cut by the board partner, and Tramiel was unwilling to take the time needed for the rework.

C64 uses the same very slow bit-banging method as VIC-20.

Same. I played with the CP/M boot disk that came with my C128 later. That was fun for a few hours. Then I went back to not knowing it existed.