VAX 85xx, 87xx, and 88xx systems

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The VAX 85xx, 87xx, and 88xx systems are a range of uni-processor and multi-processor supermini VAX systems that share being built around the NMI Memory Interconnect bus (probably the 'Nautilus Memory Interconnect', although no remaining DEC documentation gives the expansion of 'NMI'). The CPUs, main memory and I/O adapters (called 'NBIs') all connect to the NMI; 8800 family systems use the VAXBI bus for their I/O bus(es).

The 8800 family uses ECL gate arrays throughout, organized in synchronous logic. (In dual-CPU systems, the two CPUs run off a common clock.)

Although given three different model designations - VAX 85xx, VAX 8700, and VAX 8800 - the DEC internal superordinate name was "VAX 8800 Family" from the beginning:

"The VAX 8800 multiprocessor and the VAX 8700, 8550, and 8500 systems all derive from the same fundamental design. Their sustained appli­cations throughput ranges from 3.0 to 12 times that of the VAX-11/780 system. In the design process, automated tools helped to correct design bugs early. ECL technology and a two-phase clock system achieve a 45-nanosecond cycle time. Micro instructions are processed simulta­neously through four logic boxes that implement a five-stage pipeline. A high-speed memory interconnect, the NMI bus, links CPUs to memory and the I/O subsystem, which connects to VAXBI buses. Many reliability fea­tures, including extensive diagnostics, are implemented." [1]

Two Generations of VAX 8800 Family models

There are two generations of VAX 8800 Family models:

While the first-generation VAX 8700 and VAX 8800 systems supported Asymmetric Multiprocessing (ASMP) for dual CPUs, the second-generation models introduced Symmetric Multiprocessing (SMP) with up to four processors in conjunction with VMS V5.0.

Both generations employed a dedicated computer for console operations; the first-generation models used a special version of the PDP-11-based Professional 380, the second-generation ones a MicroVAX II.

VAX 8800 Family Commonalities

There are a couple of commonalities:

  • "Box Design" (first introduced with the VAX 8600)
  • Common Basic CPU Boards
  • MS88 Memory Modules
  • Microcode Format
  • System Busses

Box Design

  • N Box = Power Conditioning Box
  • E Box = Execution Box
  • C Box = Cache Box
  • M Box = Memory Box
  • I Box = Instruction Box

Common Boards

CPU Boards common to all of the VAX 8800 Family models:

  • F1002 Shifter Module
  • F1003 ALU Slice 1
  • F1004 ALU Slice 0
  • F1005 Address Data Path
  • F1009 Writable Control Store Module
  • F1011 DB88 NMI to BI Interface Module
  • F1021 Memory Controller

MS88 Memory Modules

All VAX 8800 Family models share the MS88 memory modules:

  • MS88-AA 4MB
  • MS88-CA 16MB
  • MS88-DA 64MB


All VAX 8800 Family models share the same set of buses:

  • Data-Path Buses
  • NMI = Memory Interconnect Bus
  • VBUS = Visibility Bus - a slow-speed bus used for diagnostics and initialization
  • VAXBI = Backplane Interconnect Bus

Comparison of VAX 8800 Family models

VAX 8800 Family Model Overview
First Generation Second Generation
VAX 8500 VAX 8530 VAX 8550 VAX 8700
VAX 8810N
VAX 8800
VAX 8820N
VAX 8810 VAX 8820 VAX 8830 VAX 8840
Nickname Flounder Skipjack Nautilus Polarstar
CPU Name 851BA 855BA 871BA 882BA 885BA
# of CPUs 1 1 1 1 2 1 2 3 4
Upgradable to VAX 8530 VAX 8550 None VAX 8800 None VAX 8820 VAX 8830 VAX 8840 None
Type of MP - ASMP SMP
ECL MCA 45ns
VUPS 3.2 4.2 6.5 6.5 11.4 6.5 11.4 16.8 22.2
max. # of BI buses 2 4 6
max. # of Memory modules 5 8
Console Professional 380 MicroVAX II
min. VMS version V4.4 V4.6 V4.4 V5.0

Model Differences

VAX 8500 <=> VAX 8530

The VAX 8530 is a VAX 8500 with "optimized" Microcode. There is a rumor that the optimization was to remove NOPs from the Microcode, but that hasn't bee confirmed yet.

VAX 8500/8530 <=> VAX 8550

The only difference between a VAX 8500/8530 and a VAX 8550 is a different Instruction Decode Module F1007, the VAX 8500/8530 module was the F1015 (see also: DEC Modules by bus).

VAX 8550 <=> VAX 8700/8800

  • All VAX 85xx models and the VAX 8700 are single CPU computers, but the VAX 8700 is expandable with a second CPU, which results in a VAX 8800 model.
  • All VAX 85xx models have somewhat limited expansion possibilities in comparison to the VAX 8700 / VAX 8800 models:
    • A maximum of 5 memory modules with the VAX 85xx; 8 memory modules maximum with the VAX 8700/8800.
    • A maximum of 2 BI buses with the VAX 85xx; 4 BI buses maximum with the VAX 8700/8800, and 6 BI buses maximum with the VAX 88x0 models.

VAX 8974, VAX 8978, and VAX 8842 Prepackaged Cluster Systems

There were three different prepackaged VAXcluster systems built from VAX 8700/VAX 88xx systems:

  • VAX 8974 - A VAXcluster of four VAX 8700 (later: VAX 8810)
  • VAX 8978 - A VAXcluster of eight VAX 8700 (later: VAX 8810)
  • VAX 8842 - A VAXcluster of two VAX 8820 (upgrade to VAX 8830s / VAX 8840s possible)

All three came complete with cluster hardware, storage devices, and software.

Both the VAX 8974 as well as the VAX 8978 were based on the VAX 8700 until the VAX 8810 came out in 1987, which then replaced the VAX 8700s in both the VAX 8974 and the VAX 8978.

It is not known how many VAX 8974 and VAX 8978 VAXcluster systems were ever build and sold.

There is an interesting article on benchmarks performed on the VAX 8974 and VAX 8978:
System Level Performance of VAX 8974 and 8978 Systems, in DTJ, Vol. 1 No. 5, September, 1987 (pp. 82-94 of the PDF)

You can watch a video about a VAX 8974 VAXcluster system being build up on YouTube: Digital Equipment - New York VAX 8974 Cluster Install - Jul 1987


[1] "VAX 8800 Family", Digital Technical Journal, Vol 1 No 4, February 1987.
[2] VAX Hardware Handbook. Volume 2-1986.

Further reading

External links