Decoding is easy. Decoding requires a lot of typing. (I am starting to believe this is true of ever aspect of interpreted emulation…)

For the AEJS decoder, I am creating a look-up table with every possible pattern for each instruction. It was my first idea, but I initially discarded it as “wasteful and inelegant”. Later, I accepted it as “really fast”.

I’m generating a large set of bit patterns that map to 68k opcodes, like so:

0100101011111010 -> BGND
0100101011111100 -> ILLEGAL
0100111001110000 -> RESET
0100111001110001 -> NOP
0100111001110010 -> STOP
0100111001110011 -> RTE

The current code can be found here.

Yes, it’s written in Python and yes, AEJS is a JavaScript project. There’s a reason. I see two approaches to generating the LUT (look-up table).

# Generate the LUT on launch (dynamic)

This has the advantage of being easier to debug. It also might be easier if I ever expand the emulator to other chips, like the 68020 or 68030.

It currently takes about 0.466 seconds on my machine to generate ~12,000 patterns. That means it should take less than 2.5s to generate the full LUT.

# Generate the LUT pre-launch (static)

This is what I’m currently working toward. is currently generating about 12,000 opcode bit patterns out of a max of I’m-not-sure-yet-but-slightly-less-than-65,536.

With this method, I would run one final time, pipe the output into a static JS file, and regex the contents into a JS array. The LUT would never have to be generated again, and would exist happily forever-after.

Cons: relies upon the LUT being 100% correct, as any errors would be hard to debug, and would require another run of to fix.

Pros: AEJS will launch faster without having to generate the LUT each time. If I can verify that the LUT is 100% correct, this is obviously the right approach.

I’ll continue on with the static method for now, and odds are I’ll stick with it. 2.5s is too big a price to pay.

Back to work:


(This post was copied from my old AEJS blog)