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- Just a guess, but it is microcode; some of the other bits may control other stuff on the board? The prints probably say (they are available for both the dual- and quad-width LSI-11 CPU cards. Jnc (talk) 14:28, 25 March 2018 (CEST)
- Yes, that's exactly what they do. There are 4 bits (MIB18-21) which are a 3-bit code, and an 'enable' bit (see section 22.214.171.124 'Special Control Functions' in the LSI-11 manual EK-LSI11-TM-003). When the latter is high, the other three are run into a 3->8 decoder to generate a bunch of special control signals.
- The thing I'm puzzled by, looking at the prints, is that the uROMs don't seem to have address input pins? There are Micro Instruction Bus (WMIB) pins 1-21, 4 clock phases, a uROM disable ... and that's it! The WMIB must be a dual-function (address and data) bus, and therefore tri-state (read by the uROM chips during the address phase, and driven by them during the data phase). Jnc (talk) 15:06, 25 March 2018 (CEST)