microcoding a PC into a PDP-11 (was: RE: Classic Computers vs. Classic Computing)

The idea is to write PDP-11 microcode for the PC platform, rather running an "emulator" under Windows or whatever. The Pentium would be viewed as the micro-architecture, the PDP-11 as the real machine. It would be table driven and fully expanded, using the PC memory rather extravagantly. You should be able to emulate simple instructions at the rate of about 4~8 Pentium opcodes for every PDP-11 opcode. If you rely on the Pentium MMU to trap accesses to the I/O page, you don't have to check for non-memory accesses from within the CPU model. The trap routines would emulate PDP-11 I/O, mapping it onto the PC hardware, rather than onto file I/O as in an emulator. The Pentium MMU can also be used to emulate the PDP-11 MMU. Map the PDP-11 registers onto Pentium registers, and never save them in memory except on a trap. This gives you a very, very fast PDP-11, IBM 1130, or whatever. If you can figure out a way to cause the machine to boot this "microcode" at powerup instead of Microsoft Wincrash, I argue that you could legitimately call this a PDP-11. I think I remember hearing that the IBM 360 VM OS did this. It seems a clean way of preserving classic architecture without having to mess with decayed disk drives, and without the compromises imposed by emulation. Sort of like rebuilding the Parthenon with injection-molded faux-marble columns and friezes. ;-) -- Jonathan Engdahl???????????????? Rockwell Automation Principal Research Engineer????? 24800 Tungsten Road Advanced Technology????????????? Euclid, OH 44117, USA Euclid Labs????????????????????? engdahl(a)cle.ab.com 216-266-6409