F21 docs

[Mike Simon <msimon@xxxxxxx>]

I have noted some complaints in this list about the F21 docs.

I think the criticism is unjustified. Let me tell you why:

F21 is a very simple but hard to understand processor.

The most complicating factor is the address incrementer.

Here is what I think happened. Chuck found that the simplest
fastest incrementer included an inversion with each pass. This makes the
addressing scheme hard to follow.

The instruction decoder is a little unusual but not terribly so.

Nowadays these hardware simplifications are easily dealt with in the compiler.

None the less it requires considerable thinking to 'get' it.

I have found the doc adequate to design this chip on a board including 
software.

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What might make doing a chip on FPGA useful is having separate memory managers
for processor, video, audio, and serial comms.  Pins are cheaper on FPGAs.

And 12ns Static RAM is low cost too.

A serious processor for machine control might run $600 to $800 in small 
quantities.

Simon