It turns out you can make a really fast instruction decoder in Scrap Mechanic if only you can figure out a special bit mask for each ALU operation. It turns out this is really frickity fucking hard to do by hand so I told a SAT solver to do it and it was fine until I asked for all the combinations I wanted and then... This.
out.dimacs is 36MB
c CNF file written by RustSAT
p cnf 217953 1366264
Also dimacs uses "c" for comments lol (it's just a huge file of numbers).
Also also it turns out using 100% of all 24 cores on a laptop for 8 hours makes for some heat issues so I had to limit each core to 50% utilization so it will take twice as long yay.
UPDATE: it's been 18 hours and the best it's got it 3.925%. If I linearly extrapolate, it will take 400-500 hours โ 2.5-3 weeks.
UPDATE 2: I've turned it off, as I don't have more than a weekend to just leave my laptop sitting around.
For each operation my CPU's ALU can do, I need to find a bit mask that turns it on only for the desired set of 8 bit opcodes. The bit mask is of the form
[01_].[01_][01_][01_][01_][01_][01_][01_][01_]where 0 and 1 mean the bit has to match and _ means it doesn't matter. The bit before the . is the xor of all the other bits in the opcode.The solver has to figure out both which opcodes represent each instruction and what bit masks are needed to give them the desired operations.