Wat is the oldest chip in your parts bin?

That is an amazing amount of effort especially writing your own dissambler.  The programmers of yesterday did seem to like jumping in and out of subroutines.  I formed that opinion, anyway, after seeing the dissasmbly of some Intellivision games.  And truth be told I couldn't extract enough meaning from the generated code to make any sense of it.  Best of luck if you ever resume this project again.

Here's one of a similar vintage. The carbon-loaded anti-static foam is now crumbling.

Was that the UART peripheral in their set of chips?

This next one is a very curious beast. I think the 1771 was a disc controller, but why was there a painted-over window on the top? Even stranger, there's one underneath, too. Perhaps they were programming their own chips.

I wish I had an LM109/LM309 more closely related to https://en.wikipedia.org/wiki/Bob_Widlar 

Random TLL inverter

I have several old chips in my collection, which makes a lot of sense as I started my electronics hobby/career in the early 70's.  I have several high-power vacuum tubes (from commercial FM transmitters, some up to 5000 Watts).  In IC's I have several gold cased ICs, including simple logic gates and such from, my early years in the military electronics field and many early processors and other long ago obsolete components.  I have a bunch of old germanium and silicon chips from some early work I did in electronic music and synthesizers.  I will have to see if I can dig up a specimen to share pictures. 

In my case it is the same. My most probably oldest chip is UCY7406 containing 6 invertors.

This chip we used at high school for basic logic labs, but in personal project I never used it. The oldest chip which I bought and used for leaning in my beginings is ATmega16.

The old chips I have, have maybe a few dozen transistors each... basic analog, basic digital logic gates. 

Your chips are pretty fancy.

Bit rot often kills undocumented electronics. Sad.

The code on the HD6303 is held in an external EEPROM, so that gets corrupted along with the rest of the EEPROM data at some point in the not too distant future.

I've already managed to access the contents of the EEPROM and extracted the raw HD6303 binary data from it. I've also already written a HD6303 disassembler and now have a 6303 assembly language listing to work with.

Although I generally understand the individual 6303 instructions, what I'm struggling with is following the overall program flow. Every few instructions is a branch or jump and I find it hard keep track of what is actually going on. I've worked out some stuff though and commented about a quarter of it. Not sure if I should be looking at writing a basic debugger / interpreter so as I can step through the instructions one at a time and outputting the  register contents. 

I've worked out quite a bit of the overall address space - in addition to the processor and EEPROM there are five external analogue data acquisition units each of which contain three timers. Four of those ADUs are used to fire the 12 fuel injector pulses. The address decoding needs some more investigation work as I appear to see ghost data as if multiple address locations are pointing at the same registers due to the way the decoding is being done.

I've worked out most of the comms protocol now, however there is some 'engine math' that I'm not sure about in order to be able to convert from the parameter data to human data. I've got a lot of it logged as look-up tables and plotted the curves, but would be nice to understand the math that was actually used.

I have a vintage supply of tried and true TTL logic chips and 555 timers. I wouldnt doubt some ICs are 40+ years old, but if they work, why toss them ?  Projects can be prototyped on a whim using on hand inventory. 

Does the code that the HD6303 runs reside in an external ROM?  If so, maybe it's possible to read back the contents.  Then of course you would have to find some method to do a disassembly of the raw binary.  It would be too much work for me. LOL.