Here's the data file if you want to check it out :)
http://marcansoft.com/uploads/piccpu.circ

And here's the microcode "source code". This gets turned into a ROM 
mapping the top 8 instruction bits to 16 control lines by a small Python 
script. Only 13 control lines are used so far.
http://marcansoft.com/uploads/pic_cpu_ucode.txt
The hex microcodecode ROM data is already in the .circ file above. The 
sample program ROM just runs a basic loop, using decfsz.

Note that I've basically tacked on two bits into the middle of the 
standard 14-bit instruction word. For instructions that take an f-reg, 
one bit is used as an extra addressing bit (this is part of the lower 
word, not mapped into the microcode). For instructions that take an 
8-bit literal, this is just a don't care (though it's inserted at a 
different spot in the instruction word - here, the full lower word is 
the literal, and the upper word has two don't care bits). The other bit 
is unused in both of these cases. With instructions that load the PC, 
these bits are used to provide 12-bit immediate PC addressing. The 
opcodes are otherwise the same. This should make it fairly trivial to 
write a converter to change a PIC hex file into this variation.

I've found that 181 ALUs are very flexible. Though they're pretty 
cryptic to work with, they can implement almost every PIC ALU 
instruction with no extra work. Only rrf needs an external mux to deal 
with the rotation (rlf is just f+f, which the 181 ALU can do). A 3-to-8 
decoder hooked up to the ALU input takes care of the b*f/btfs* 
instructions as well. I haven't done swapf, since it would involve 
another output MUX like rrf, and it isn't really used all that often.

Hope you find some use for it! I've found watching a "PIC" run through 
code and seeing every signal to be pretty entertaining and educational, 
if nothing else. I've showed it to other non-engineers, and they do get 
a quick idea of the linear, step-based nature of sequential logic 
systems like CPUs, just by watching the lines flicker on and off as the 
PC steps through the code.

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

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