PDP-11/74

INTRODUCTION

The PDP-11/74 was an interesting multi-processor development that never saw successful commercial deployment. Some on-line articles describe this as an experimental system, but a lot of effort and financial resources must have gone into building this PDP-11/70 variant. However, by the time the system and its new operating system (RSX-11M Plus) were ready, the VAX was making its commercial appearance.

There are various references to the PDP-11 multiprocessor. Some literature refers to is the 11/70mP, the 11/72 (denoting two tightly coupled processors) and the 11/74 (four processors). The 11/70mP could be configured as a quad, dual or single CPU. The multiprocessing configuration was technically solid.

PDP-11/74 System

However, there was quite a lot of overheard at DEC to configure these systems. A DEC working group configured decided on the configuration of each system delivered. Reliability was also a concern for this complex configuration. The failure of one CPU could bring down all other CPUs. The problem with the 11/74 was maintenance. Diagnostic loading required that the machine have part of the shared memory available for testing and the many long memory box cables were a maintenance nightmare. DEC felt the field technicians weren't up to maintaining them and declared the PDP-11/74 as non-maintainable.

Rather than continue to sell a 16-bit system, DEC focused on positioning the VAX 11/780 as the preferred platform. One DEC representative (Christine M.) claimed nobody at DEC was allowed to tell customers about these systems as it might compete with the VAX. There are references on-line about the PDP-11/74 being much faster than the VAX and having a significantly higher performance this could cannibalize VAX sales. However, this does not seem likely as the VAX was the future 32-bit architecture as compared to the legacy PDP-11 16-bit architecture and only 4MBytes of memory as opposed to the VAX 4GByte virtual memory space.

The strength of the 11/74 was it's implementation of the Commercial Instruction Set (CIS) which was reputed to give it a much better performance over the VAX-11/780s CIS.

When DEC canceled this system, the few PDP-11/74s were divided into individual PDP-11/70s and sold to clients or used in-house. One technician at Princeton University running XXDP+ diagnostics on their PDP-11/70 noticed that the CPU was a KB-11CM; a PDP-11/74 CPU. The 11/74s MK11 MOS memory was used in other 11/70s, the VAX-11/730 and VAX-11/750. A remaining PDP-11/74 was used by DEC until 1986 for RSX development.

ARCHITECTURE

This was a tightly coupled symmetric multi-processor version of the PDP-11/70. The system consisted of two to four PDP-11/70s interconnected, with some minor tweaks made to the CPU. The CPU was the KB11-E variant, being based on the KB11-CM. An interprocessor interrupt and sanity timer (IIST) facility in each CPU interconnected over a private bus, allowed one CPU to interrupt or bootstrap another CPU. The ASRB CPU microcoded instruction implementation was modified to be "atomic" (a semaphore), for use in synchronization between CPUs. DT07 (UNIBUS switch control) and PCL11s (communications link) were used for inter-processor links (ref: Mike C.).

Changes to the KB11 microcode were kept to a minimum due to the microcode address space being only 9 bits wide. As well, there already wasn't much available space in the microcode ROMs to add additional instructions.

The MKA11 multi-ported memory was used to connect the processors. As the memory was shared amongst four processors (for an 11/74) the actual performance for a four processor 11/74 configuration was equivalent to three 11/70s due to memory contention. High speed devices resided on the MASSBUS. Most of devices such as disk and tape drives were dual ported and could be shared between two CPUs. Other devices on the UNIBUS could not be shared unless a UNIBUS bus switch (DT03/DT07) was installed.

Each PDP-11/70 CPU in the multiprocessor configuration had its own physical console. Each CPU had it's own UNIBUS for I/O. If I/O to a unique device had to be done and that device was on one of the four CPUs, then that CPU had to perform the I/O. DT03 and DT07 UNIBUS switches were also used to share peripherals between different CPU buses.

PDP-11/74 Cabinet Layout

CIS

The 11/74 featured a high performance implementation of the Commercial Instruction Set, supporting character string moves, decimal arithmetic and other features. Three hex size logic boards were added to the backplane to support CIS. Some MASSBUS slots were removed for CIS hardware to maintain the backplane the same physical size as the 11/70s. CPUs were updated with new logic to access the CIS hardware.

The CIS option for the PDP-11/44 was implemented at the same time as that of the 11/74. However, the 11/44 CIS performance was much lower than that of the 11/74. The 11/44 CIS implementation was over two boards; a hex datapath M7092 in slot 2 and a quad control store board in slot 1 of the 11/44 backplane.

PDP-11/74 Maintenance

RSX-11M Plus

RSX-11M was modified to handle the multi-processor configuration; RSX-11M Plus. If anyone looks into the RSX Plus code they will see references to the multi-processor enhancements added into this operating system. The biggest changes were to accommodate I/O on different CPUs. An I/O operation on a device on a different CPU causes RSX to fork the I/O request to the CPU with the device.

CLIENTS

One DEC user recalls that only four of these systems were made with two being given to customers. One was given to a bank in New York city (Bankers Trust?) that had a already large base of installed PDP-11/70s. Another system was shipped to Ontario Hydro (Canada), also an existing user of PDP-11/70s. One system was built for AT&T but not sure if it was ever shipped. One system was made for the Alberta Government Telephones (AGT) company, now Telus, but was later returned to DEC (ref: Garry L.)

PDP-11/74 System Under Maintenance

GUIDES

Document Name	Order Part No.	Publication Date	Domain
PDP-11/74 Multiprocessor System Evaluation	N/A	21 June 1979	HW
PDP-11/70 Multiprocessor Technical Manual (Preliminary)	EK-70MP-TM-PRE	1977	HW
Introduction to Multiprocessing (Inter Office Memo)	N/A	August 1980	Sales
Preliminary 11/74 KB11-E Difference Specification (Inter Office Memo)	N/A	15 August 1978	HW
RSX-11M Multiprocessing Software Functional Spec. (Inter Office Memo)	N/A	20 April 1977	SW

Sources:

• Adapted from: Wikipedia PDP-11
• Adapted from: BitSavers, PDP-11/74
• Adapted from: Gunkies PDP-11/74
• Adapted from: oboguev.livejournal.com (11/74 pictures)

Compiled on 07-24-2024 16:10:26