Extra chips in PDP11/23 plus cpu board

[Nigel Williams]

On Wed, Dec 20, 2017 at 4:45 PM, Douglas Taylor via cctalk
<cctalk at classiccmp.org> wrote:
> There is a CPU board for sale on ebay, M8189, and it has the usual 3 chips
> CPU, MMU, FPU.  However, there are 2 extra chips and I've never seen that
> before.

https://en.wikipedia.org/wiki/PDP-11_architecture#Optional_instruction_sets

Microcode ROMs for the CIS (Commercial Instruction Set)