This I would like to know more about. Last year I started to get into electronics by creating on my own synth using an Arduino. As a software boffin, it was marvellous to learn about PWM, timers and the MIDI protocol (along with everything else). Obviously, I read up on the various synths over the years to see how they did their sound generation.
Where do you even start with your project? How to you even get to the disassembled code?
Dump the ROM through an EPROM programmer used to read back the data, then try to figure out what chip it is (found a schematic somewhere), turns out there are two CPUs in it, one which is mask programmable which handles all the A/D and the buttons and keys, the second one which drives the whole thing. The two CPUs talk on some funky parallel link (think 'LapLink' but then for two old 8 bitters sitting 5 cm away from each other).
After you know the CPU it is relatively straightforward, you try to figure out the memory map by looking at the address decoding circuitry , and from there you can infer things like where the vectors are pointing (usually they are at the end of the memory map in CPUs from that era, alternatively at '0'). So that will give you a good idea where the ROM should be located, (in this case from $C000 upwards).
Then you spend lots of evenings with cups of tea and some quiet music on trying to figure out what all the instructions are trying to achieve. A good starting point is any ASCII strings, then tables of pointers to code and strings, then trace the boot vector until you come to the inevitable main loop and trace inwards from there. IRQ and NMI vectors are also useful entry points to dig around in (NMI not used here). It's a job that requires quite a bit of patience and a false start somewhere can cost you dearly. I love the little 'aha!' moments whenever I've sussed out what a particular bit of code does. For instance, the DX7 can display the battery voltage and the patch numbers and other information in decimal, like it should. So then you come across this repeated subtraction test with the contents of a table, turns out it is the binary-to-decimal convertor.
It's going to be a while before this is done but it is very interesting.
Brilliant comment, this is what I come to HN for. I'm starting to get into electronics myself and as a developer I fear it's only a matter of time until I get sucked into this side of things...
Where do you even start with your project? How to you even get to the disassembled code?