KDF11 CPUs

There are several single-board CPUs which all use the 'Fonz' F-11 chip set:

• KDF11-A CPU - M8186 - QBUS dual-width CPU used in the PDP-11/23
• KDF11-B CPU - M8189 - QBUS quad-width CPU used in the PDP-11/23+
• KDF11-U CPU - M7133 - UNIBUS hex-width CPU used in the PDP-11/24

Like the LSI-11 models, as a cost-reduction measure they do not have a front panel to control them; instead, when the CPU is halted, specialized microcode used the main serial line as a operating console. The command set is named Octal Debugging Technique (ODT); there are commands to read and write memory, start the CPU, etc. The main serial interface is normally configured so that when the CPU is running, sending a break on the console serial line halts the CPU.

Floating point

All the KDF11 CPUs have two choices for floating point support (full PDP-11 FP11 floating point): a on-board single chip, the KEF11-A floating point chip, which implements floating point using microcode; and a higher-performance separate quad board, the FPF11 (M8188).

The FPF11 communicates with the KDF11 via a cable that plugs into the socket on the KDF11 where the KEF11-A is installed; is unusual that it can plug into either a QBUS or UNIBUS backplane, since it only draws power from the backplane - all signals come over the cable to the KDF11.

ODT limitations

The F-11 chip set includes microcode which provides 'front panel' functionality named 'ODT'; the ability to read and write to memory, start the process, etc. However, the original version of the KDF11-A only supported 18-bit addressing, and even though later versions supported 22-bit addressing, ODT in the KDF11's was always limited to 18-bit addressing: i.e. it is impossible to interact with memory above 256 Kbytes from ODT.

The later KDJ11 CPUs do not have this limitation.