disassembling Western Digital microcode (PDP-11, WD16, Pascal Microengine)

The first cut at a microcode disassembler for the CP16xx/WD21xx chipsets, written in Python 3, is now on github: https://github.com/brouhaha/cp16dis The disassembler uses hexadecimal, in C notation, rather than octal as used by DEC. Branch targets are labeled as L with the three-digit hex address. The disassembler doesn't know about branch targets only reached from the control chip translation PLA. The disassembler probably needs a lot of work to be truly usable. It does not even attempt to produce code that could be assembled by DEC's "MICRO" microassembler that is in the KUV11 writable control store support software, or any other assembler or microassembler. (I don't have the KUV11 support software, but would *really* like to obtain a copy!) The register names decoded are specific to the LSI-11 microcode. I don't yet know what the Pascal Microengine registers are used for, so I don't have suitable names for them. Here's an excerpt from the LSI-11 microcode at address 0x018 where you can see one of the techniques for a computed jump, with entry occuring at 0x20: L018: jmp L101 ;018: 000101 jmp L161 ;019: 000161 jmp L192 ;01a: 000192 jmp L0f5 ;01b: 0000f5 jmp L0a5 ;01c: 0000a5 jmp L19e ;01d: 00019e jmp L192 ;01e: 000192 jmp L192 ;01f: 000192 mi RIRL,RIRH ;020: 00ec89 jmp L018 ;021: 000018 This is used by an implicit jump caused by the translation array in the control chip to location 0x20. The mi instruction causes the instruction register (RIRH) to be OR'd into both bytes of the next microinstruction, which is a jump to L018. Presumably before jumping to the mi, all of the bits of the instruction register have been masked off, except for the least significant three. It's also possible that it's been shifted before masking. Also possible, not all of the bits might be masked off, and there might be more of the jump table elsewhere, depending on those bits. Note that if the most significant five bits are not masked off, this could even change the jump instruction at 0x21 to a different kind of instruction. Eric