OT-ish: Build-it stuff

Having unknown hardware in your hands isn't, IMOHO, nearly as bad as unknown software. Consider my conundrum right now... I've got a diskette with software (no documentation beyond how to stick it in the slot on the machine) for a computer that a customer wants to emulate on a PeeCee, the last remaining sample of the original hardware having flown the coop. No engineering documents at all. The instruction set (from the disk) appears to be 8086, but the OS is unknown to me and from the looks of it, the disk is loaded into memory as a single 240K image. I can figure out the load address by explicit segment references. From the very few ASCII text strings in the code, this is apparently a multithreaded, interrupt-driven OS. or some-such) readout and a diskette drive with an RS-232 port. Otherwise, ASCII searches yeild no error messages, prompts, or anything that would allow me to quickly determine control flow. I have absolutely no idea of the peripheral configuration. This is circa 1981-82. So, where do I start? I could put it on an emulator and attempt to trace program flow, but given the nature of the OS (event-driven) it's not going to be easy. I can locate a few ISRs, and from their appearance, the box doesn't use an 8259 PIC (no EOI sequences at the end of the ISRs). If this were a hobby thing, I wouldn't care about the time I put in--but the customer wants a quote. Any suggestions? Cheers, Chuck