It was originally conceived as a multi-purpose synth chip. Bob Yannes, designer: "When I designed the SID chip, I was attempting to create a single-chip synthesizer voice, which hopefully would find it's way into polyphonic/polytimbral synthesizers. I never realized this however."
Thanks both. I was relying mostly on the existence of the SIDStation some years back.
Emulated, but from what I understand, not perfectly. Mainly because the SID was a hybrid digital and analog IC - and it's that analog part that made it unique.
Also - I don't think (I could be wrong) that any successful reverse engineering of the chip was ever done, nor do I think that there were any or much documentation about the chip (that is, official docs)? It was very much one of those custom chips made for one specific purpose that wasn't used in any other product (this was a common thing in many home computers of the 1980s).