DEC, PDP-11/44 #1


The Digital Equipment PDP-11/44 is the last of the PDP-11s to be designed with discrete semiconductors. All subsequent models were based on microprocessors. It was intended to be a lower cost replacement for the PDP-11/70 and PDP-11/45 systems. The 11/44 has 8K of cache memory to improve performance, and separate Instruction and Data address space, an advanced design for the time. It can run relatively modern versions of the UNIX operating systems.


An FP11-F Floating Point Processor (FFP) and KE44-A Commercial Instruction Set Processor (CISP) can be added to accelerate scientific or business applications. Our PDP-11/44 has the FFP installed.


This unusual design includes an 8085 microprocessor on the Multifunction Module (MFM). The MFM performs the OS boot, provides the serial console port and a second serial port that can be used for diagnostics or for a TU58 tape drive. The 8085 microprocessor makes diagnosing system problems much easier than on other PDP-11s.


The PDP-11/44 KD11-Z processor has a 22 bit memory address so it can be equipped with 4 MB of RAM. There are four Extended-Unibus slots in the main 14-slot backplane that support the extended 22-bit address. The Unibus in the remainder of the system only supports an 18-bit address.




We installed an M8256 RX211 floppy disk controller in slot 14 of the main backplane. This is connected to the RX02 floppy disk subsystem.


We added a DD11-DK 9-slot Unibus backplane to this system to provide slots for more I/O boards.

We installed M7161-YA and M7162 modules that make up the UDA-50 SDI disk controller in this backplane.


Our PDP-11/44 includes two RA81 disk drives that are currently non-functional.


A blog on the restoration of this PDP-11/44.