KEF11-B CIS chip
From Computer History Wiki
The KEF11-B CIS chip is an option for some PDP-11 CPU boards which use the F-11 chip set; it implements the PDP-11 Commercial Instruction Set.
It consists of six chips containing additional microcode, on a large dual DIP carrier. Due to its large physical size, not all the KDF11 CPUs can use it and the KEF11-A floating point chip at the same time; the KDF11-A CPU is too small to hold them both simultaneously.