DEC PDP-11/20

S/N S-1294

This PDP-11/20 was donated by Thomas Holmes on 12/19/23.

The PDP-11/20 was the first 16-bit system built by DEC and was introduced in January 1970. It was the first of the PDP-11 family, and the first to use the Unibus I/O bus. There are eight general purpose registers in the processor that can be used as accumulators, pointers, or index registers. Register R6 is the Stack Pointer, and R7 is the Program Counter. Because it has a Stack Pointer it supports reentrant code. The instruction set and addressing modes are much more complex than the earlier PDP-8 and PDP-9 minicomputers. The base system had no memory management, so the 16-bit address limited the memory to 32k words (64k bytes). The chassis could be a BA11-CC Table Top, or a BA11-CS Rack Mount , both with an H720 Power Supply. A BA11-EC or BA11-ES Table Top of Rack Mounted Extension Mounting Box could be connected to the processor chassis to provide room for more Unibus devices. A year after the introduction the KT11-B Paging option was introduced that increased the address size to 18-bits and the maximum memory to 256k bytes.

The system originally shipped with just paper tape based software development tools. Later a disk based DOS operating system and tools were available. A PDP-11/20 equipped with the KE11-A Extended Arithmetic Element and an RF11/RS11 256K-word fixed head disk could run UNIX V1.

The PDP-11/20 is built upside-down like the PDP-8/L system. Later PDP-11 systems had the backplane at the bottom of the chassis. At the far right are three 4-slot backplanes wired for the KA11 processor. The next 4-slot DD11A backplane is empty and can be used for small peripheral controller boards. The two 4-slot MM11 backplanes at the left each hold 4k of MM11-F core memory.

The following boards are installed in the chassis.

Slot P/N Description











This is the KA11, PDP-11/20 Processor. The M821 was in the wrong slot and was moved to slot 11.



These are the two MM11-F 4k x 16-bit core memories.