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

Yes I do. Although I never actually implemented a microcoded machine, my co-workers at Allen-Bradley were building them in the late 70's and early 80's. I studied micro-programmed architectures extensively at that time, did several paper designs, and wrote the microcode for them. Unfortunately, building the hardware at the time was prohibitively costly and difficult to do on my own time/nickel. By the time I got the company to put one of my machines into production, it was a two stage pipelined RISC, with jump slots and all that, like a MIPS. No microcode at all. Every instruction was hard-wired, and executed in a single cycle. The Allen-Bradley PLC-3, and the later PLC-2 were both AMD 29xx based microcoded architectures. The early PLC-2, the mini-PLC 2, and the PLC-5 were microcoded machines also. The early PLC-2 used an 8080, and the mini PLC-2 used a Z-80 as the micro-engine. Weird techniques were used: mapping ROMs, hardware assists for the boolean opcodes, etc. On the mini there was no hardware assist except for the mapping ROM. In micro-engine mode, the stack pointer was used as the PC of the emulated machine. An opcode fetch was something like a POP HL followed by a couple LDs and a JP (HL). During the pop the opcode byte passed through the mapping ROM to turn it into the address of the execution routine for that opcode. When an interrupt occurred the Z-80 operated more traditionally. A Pentium microprocessor is just as good a microengine as an AMD 2901/2910, or a 3000 series bit slice. The techniques I intend to use are the same as those used in the microcoded machines of the 70's, except there is no specialized hardware assist for decoding instructions, such as the mapping ROM, or address mode decoders. These can be replaced by large, fully expanded tables in RAM, now that RAM is so dirt cheap. That's not what I'm saying. The Pentium is probably not microprogrammed anyhow, at least for the majority of instructions. They probably have a microcoded front end for the messy stuff, like context switching. I'm adding one more level of abstraction -- microcoding on the microcoded machine, so to speak. Like the Mini PLC-2. Yes, a microcoded machine is one kind of emulator. Every microcoded architecture is an emulation. The underlying micromachine bears little resemblance to the machine that the user sees. It's a nice distinction, and not razor sharp. I would say that an "emulator" runs under an OS on a machine that is still running windows, NT, Linux, or whatever. A microcoded machine runs barefooted on the hardware, over which it has exclusive control. Whether that hardware itself is microcoded makes no difference. It is so. I don't know of any rule that says that the micromachine has to have a particular type of architecture, or that it can't be an off-the-shelf machine, rather than a collection of AMD 29xx parts. You can argue that hardware is software, or that software is hardware. It's all logic. The distinction is pretty academic. What I am saying is that it would be possible to re-create almost any classic architecture using much the same techniques used back in the 70's to create the original machines. Instead of designing a custom micromachine, you just go down to the corner store, buy a Pentium IV machine, erase the hard drive, and throw away the disks. Now, on another machine, sit down and write the "microcode" in x86 assembly language, load it on a floppy, and boot the Pentium IV, which now becomes a PDP-11, 1130, CDC, or whatever. If you copy a disk image of the target OS to the IDE hard drive, it will boot and run just like the original machine. -- 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