Well if you don't know what the processor is, then presumably you don't know what the hardware is either?, so you haven't much hope of disassembling it.
Assuming you can identify the processor, then you should be able to find a dis-assembler for it, and generate crude assembler source code.
I included a dis-assembler in PicProg and WinPicProg, long ago, but PIC assembler been RISC is fairly easy to dis-assemble.
As it's such an old EPROM (and the fact it's an EPROM at all) it's probably for a micro-processor rather than a micro-controller, but there's a vast number of processor options - Z80 might be worth trying?.
As others have said, you can't generate C code from it, and it probably wasn't written in C anyway depending how old it is?
What are you trying to do anyway?