[TUHS] Encoding an ISA: Random Logic vs. Control Stores

[Ron Natalie]

There seems to be confusion here about microcoding vs. discrete 
logic/LSI.    All but the initial 11/20 (and its variants) are 
microcoded.   That is there is a programmed sequence of operations in 
the execution of the individual instructions.   The less popular 11/60 
even featrued a writable control store.

All of the early PDP-11s were built out of discrete logic.   Relatively 
simplistic combinations of various logic chips, gates, flip flops, etc.. 
   A lot of these were encapsulated into the DEC "Flip Chips" small 
circuit boards to do common sets of operations.   While this is by 
modern standards, crude, it doesn't preclude microcoding and other 
advanced tasks.    I worked on a supercomputer built that way (Denelcor 
HEP built largely out of Motorola 10800-series ECL chips).

As I previously stated, the 11/44 was the last of the discrete logic 
PDP's in 1979.    The later systems were all large scale integrated 
circuit based.  First starting with the LSI-11/03 on the Q bus, quickly 
followed by the 11/23 and the Unibus 11/24.