[sdiy] Analysis of the TB-303 CPU timing

rsdio at audiobanshee.com rsdio at audiobanshee.com
Mon Mar 20 12:11:01 CET 2017


There’s no need to chase three or four different disassemblers. All you need is the data book for the exact processor part number, and it will list a definitive set of opcodes. You’re correct that a generic disassembler might get some opcodes wrong, but then your listing will quickly turn into garbage, and that should be blatantly obvious. For these 8-bit processors, it’s usually a very quick task to check a few tens of opcodes (there are usually less than that, and not more than 256, of course). There is a one-to-one mapping between machine code and assembly code.

There is no such thing as disassembler optimization. You have it backwards. When code is written in a high-level language like C, the compiler can create different opcodes for the same source depending upon the optimization level and the compiler vendor. However, when you’re looking at assembly or machine code, there is no optimization layer. What you see is what the processor will execute. In fact, the timing is precisely known when looking at machine code or disassembly of machine code into assembler syntax.

So, the trouble here is not the disassembler, but recreating the original source code that the developers used. For something of the TB-303 era, the developers probably wrote the entire firmware in assembler. Writing firmware in C was not very common, especially when the ROM was only 2 KB. That’s fewer than 2048 instructions, because many instructions have operands that take an extra byte or two, and then there are the tables of data that should not be disassembled.

Uncommented assembly is very difficult to read because there are no variable names or comments. It has nothing to do with optimization and everything to do with human-readable code. The processor doesn’t need the comments, so the ROM doesn’t include them.

I write my own disassemblers for projects like this one. I do that because I can embed symbols that represent the custom hardware of the particular vintage synth in question. That means my disassembler listing is more human readable (at least for one particular human) than the default disassembler.

Basically, you’re right: Don’t trust a disassembler unless you verify its translation against the official data book for the CPU. But optimization is not part of the process of disassembly. That’s in the realm of compilers. You’ll be able to determine the precise timing from the ROM data and the data book.

Brian Willoughby
Sound Consulting


On Mar 20, 2017, at 3:50 AM, KRiSh <krish0005 at gmail.com> wrote:
> as far as i remember, disassemblers are not guaranteed to output exactly the original code.
> 
> it depends how the disassembler work and only defining the processor family or model could not be enough..
> 
> you might obtain a very similar version of the original code, but not as optimized as the original nor 100% identical.
> 
> any "optimization" is of course useful in the context of each corresponding project,
> and we are talking about a 1981 project so, probably there where some..
> 
> i understood the thread is to try re-creating the original source code and understand deeply how the 303 seq works,
> especially including any quirk that might contribute to its disputed "magic" or "unique" feel.
> 
> If that's the objective, then i assume more work is needed on the pre-requisites in this case.
> 
> I would compare at least 3/4 different disassemblers output and see if the code resulting code is the same.
> 
> Is it the same ?
> 
> If not what is different ?
> 
> which disassemblers have been used so far ?
> 
> Trusting your disassembler 100% and use the resulting code being confident it is already 100% equal to original is a wrong assumption IMHO that could lead you to waste your time (in this particular case).
> 
> If recreating also the quirks is important here, then you really need the correct original code to understand and identify all the interactions of the routines/interrupts and their timings, especially because efficiency of code and, as said any "optimization", would be very important or crucial to the resulting timing.
> 
> IMHO, just my 2 cents, etc
> 
> K





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