Talk:LSI-11

Microcode width

Question: How come the microcode ROMs are 22 bits wide, yet the opcodes are 16 bits wide? (AS documented in the WP16/CP1600 manuals.) Larsbrinkhoff (talk) 11:16, 25 March 2018 (CEST)

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 4.2.2.7 '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)

Thanks a lot! I think it would be intersting to make an interpreter for the uROM. Larsbrinkhoff (talk) 22:21, 26 March 2018 (CEST)