PDP-11/45, /50, /55


Introduced two years after the /20, in June 1972. The KB11 was a faster, microcoded CPU built with SSI/MSI (Small/Medium Scale Integration) components. The machine had two different buses: one was a UNIBUS with 18-bit addressing, the other was a fast CPU-memory interconnect. It also introduced split I/D (Instruction/Data) spaces (UNIX used this; the DEC operating systems did not), an MMU (Memory Management Unit) option, an optional FPU (Floating Point Unit). The maximal ammount of memory was 128 Kwords.

The frontpanel was even nicer than the /20's.
A really wonderful picture of the frontpanel (thanks to Csaba Tóth!)
A nice configuration
The "mysterious" /45 

11/45 at the Technical University of Budapest: in the foreground you can see a Videoton VDT52100 (VT52-compatible) terminal.

The cycle time of the /45 with bipolar memory (max. 8 KW!) was 300 ns, MOS memories (max. 32 KW) were 450 ns, and core was 980 ns - but without memory management! The MMU added 90 ns to the cycle time.

The /50 was basically the same machine with different memory. The /55 (KB11D) used the modified CPU of the /70, where the cache was left out (instead the memory was the faster bipolar memory). It also had the kernel/supervisor/user operating modes seen on the /70, but only had a 18-bit addressing range. It was the fastest of the "classic" -11 CPU's when measured by the cycle times.

A PDP-11/50 processor
A full PDP-11/55 system