Thanks. I wonder why I'd never heard of it. The 4052 has a 128K address space - separate 64k spaces for code and data. That said, there are ways of running code in the data space and accessing data in the code space. Yes, much. The service manuals tell you a lot about how the microcode is executed, but little about what's in there. Similarly there is a list of instruction mnemonics and addressing modes against opcodes, and even an explanation of how the addressing modes work, but nothing to tell you what the instructions do, let alone anything in the way of ROM listings. One day I might get around to decompiling the microcode... Philip.