The MicroVAX was a family of low-cost minicomputers developed and manufactured by Digital Equipment Corporation (DEC). The first model, the MicroVAX I, was introduced in 1983.[1][2] [3] They used processors that implemented the VAX instruction set architecture (ISA) and were succeeded by the VAX 4000.

MicroVAX I

The MicroVAX I, code named "Seahorse",[4] introduced in October 1984, was one of DEC's first VAX computers to use very-large-scale integration (VLSI) technology. The KA610 CPU module (also known as the KD32) contained two custom chips which implemented the ALU and FPU while TTL chips were used for everything else. Two variants of the floating point chips were supported, with the chips differing by the type of floating point instructions supported, F and G, or F and D.


The MicroVAX II, code named "Mayflower", was a mid-range MicroVAX introduced in May 1985 and shipped shortly thereafter.[5][6] It ran VAX/VMS or, alternatively, ULTRIX, the DEC native Unix operating system.

It used the KA630-AA CPU module, a quad-height Q22-Bus module, which featured a MicroVAX 78032 microprocessor and a MicroVAX 78132 floating-point coprocessor operating at 5 MHz (200 ns cycle time). Two gate arrays on the module implemented the external interface for the microprocessor, Q22-bus interface and the scatter-gather map for DMA transfers over the Q22-Bus. The module also contained 1 MB of memory, an interval timer, two ROMs for the boot and diagnostic facility, a DZ console serial line unit and a time-of-year clock. A 50-pin connector for a ribbon cable near the top left corner of the module provided the means by which more memory was added to the system.

The MicroVAX II supported 1 to 16 MB of memory through zero, one or two memory expansion modules. The MS630 memory expansion module was used for expanding memory capacity. Four variants of the MS630 existed: the 1 MB MS630-AA, 2 MB MS630-BA, 4 MB MS630-BB and the 8MB MS630-CA. The MS630-AA was a dual-height module, whereas the MS630-BA, MS630-BB and MS630-CA were quad-height modules. These modules used 256 Kb DRAMs and were protected by byte-parity, with the parity logic located on the module. The modules connected to the CPU module via the backplane through the C and D rows and a 50-conductor ribbon cable. The backplane served as the address bus and the ribbon cable as the data bus.

The MicroVAX II came in three models of enclosure:

  • BA23
  • BA123
  • 630QE - A deskside enclosure.


KA620 referred to a single-board MicroVAX II designed for automatic test equipment and manufacturing applications which only ran DEC's real-time VAXELN operating system. A KA620 with 1 MB of memory bundled with the VAXELN Run-Time Package 2.3 was priced at US$5,000.[7]


Mira referred to a fault-tolerant configuration of the MicroVAX II developed by DEC's European Centre for Special Systems located in Annecy in France. The system consisted of two MicroVAX 78032 microprocessors, an active and standby microprocessor in a single box, connected by Ethernet and controlled by a software switch. When a fault was detected in the active microprocessor, the workload was switched over to the standby microprocessor.[8]

Industrial VAX 630

A MicroVAX II in BA213 enclosure.


BA23- or BA123-enclosure MicroVAX upgraded with KA650 CPU module containing a CVAX chip set.


BA23- or BA123-enclosure MicroVAX upgraded with KA655 CPU module.


BA23- or BA123-enclosure MicroVAX upgraded with KA660 CPU module.

MicroVAX 2000

The MicroVAX 2000, code named "TeamMate", was a low-cost MicroVAX introduced on 10 February 1987.[9] In January 1987, the MicroVAX 2000 was the first VAX system targeted at both universities and VAX programmers who wanted to work from remote locations.

The MicroVAX 2000 used the same microprocessor and floating-point coprocessor as the MicroVAX II, but was feature reduced in order to lower the cost. Limitations were a reduced maximum memory capacity, 14 MB versus 16 MB in MicroVAX II systems and the lack of Q-Bus or any expansion bus. The system could have two Winchester drives and had a built in 5.25-inch floppy drive for software distribution and backup. Supported operating systems were VMS and ULTRIX.[9] It was packaged in a desktop form factor.

MicroVAX 3100 Series

The MicroVAX 3100 Series was introduced in 1987. These systems were all packaged in desktop enclosures.

MicroVAX 3100 Model 10
Teammate II
KA41-A, CVAX, 11.11 MHz (90 ns)
MicroVAX 3100 Model 10e
Teammate II
KA41-D, CVAX+, 16.67 MHz (60 ns)
32 MB of memory maximum.
MicroVAX 3100 Model 20
Teammate II
KA41-A, CVAX, 11.11 MHz (90 ns)
A Model 10 in larger enclosure.
MicroVAX 3100 Model 20e
Teammate II
KA41-D, CVAX+, 16.67 MHz (60 ns)
A Model 10e in larger enclosure.
MicroVAX 3100 Model 30
Entry-level model, developed in Ayr, Scotland[10]
Introduced: 12 October 1993
KA45, SOC, 25 MHz (40 ns)
32 MB of memory maximum.
MicroVAX 3100 Model 40
Entry-level model, developed in Ayr, Scotland[10]
Introduced: 12 October 1993
KA45, SOC, 25 MHz (40 ns)
8 to 32 MB of memory
A Model 30 in larger enclosure.
MicroVAX 3100 Model 80
Entry-level model, developed in Ayr, Scotland[10]
Introduced: 12 October 1993
KA47, Mariah, 50 MHz (20 ns), 256 KB external cache
72 MB of memory maximum.
MicroVAX 3100 Model 85
Introduced: August 1994[11]
KA55, NVAX, 62.5 MHz (16 ns), 128 KB external cache
16 to 128 MB of memory.
MicroVAX 3100 Model 88
Introduced: 8 October 1996[12]
Last order date: 30 September 2000[13]
Last ship date: 31 December 2000[13]
KA58, NVAX, 62.5 MHz (16 ns), 128 KB external cache
64 to 512 MB of memory.
MicroVAX 3100 Model 90
Introduced: 12 October 1993
Identical to the VAX 4000 Model 100, but uses SCSI instead of DSSI
KA50, NVAX, 72 MHz (14 ns), 128 KB external cache
128 MB of memory maximum.
MicroVAX 3100 Model 95
Introduced: 12 April 1994[14]
Processor: KA51, NVAX, 83.34 MHz (12 ns), 512 KB external cache.
MicroVAX 3100 Model 96
KA56, NVAX, 100 MHz (10 ns)
16 to 128 MB of memory.
MicroVAX 3100 Model 98
Introduced: 8 October 1996[12]
Last order date: 30 September 2000[13]
Last ship date: 31 December 2000[13]
KA59, NVAX, 100 MHz (10 ns), 512 KB external cache.
InfoServer 100/150/1000
General purpose storage server (disk, CD-ROM, tape and MOP boot server) related to MicroVAX 3100 Model 10, running custom firmware, KA41-C CPU.[15]


MicroVAX 3500 and MicroVAX 3600

The MicroVAX 3500 and MicroVAX 3600, code named "Mayfair", were introduced in September 1987 and were meant to be the higher end complement of the MicroVAX family. These new machines featured more than three times the performance of the MicroVAX II and supported 32 MB of ECC main memory (twice that of the MicroVAX II). The performance improvements over the MicroVAX II resulted from the increased clock rate of the CVAX chip set, which operated at 11.11 MHz (90 ns cycle time) along with a two-level, write-through caching architecture. It used the KA650 CPU module.

MicroVAX 3300 and MicroVAX 3400

The MicroVAX 3300 and MicroVAX 3400, code named Mayfair II, were entry-level to mid-range server computers introduced on 19 October 1988 intended to compete with the IBM AS/400.[16] They used the KA640 CPU module.

MicroVAX 3800 and MicroVAX 3900

The MicroVAX 3800 and MicroVAX 3900, code-named "Mayfair III", were introduced in April 1989.[17] They were high-end models in the MicroVAX family, replacing the MicroVAX 3500 and MicroVAX 3600, and were intended to compete with the IBM AS/400. At introduction, the starting price of the MicroVAX 3800 was US$81,000 and that of the MicroVAX 3900 was US$120,200.[18] A variant of the MicroVAX 3800, the rtVAX 3800, was intended for real-time computing (RTC) applications such as computer-aided manufacturing (CAM). These systems used the KA655 CPU module, which contained a 16.67 MHz (60 ns cycle time) CVAX chip set. They supported up to 64 MB of memory.

Operating systems

VAX/VMS was the native operating system for the MicroVAX series, though earlier versions required a specialized version designated MicroVMS.


MicroVMS was, for a while, a separate product[19] with its own distinctive manuals, and it began with "tackling the technical problems one by one" (beginning with) the limitations imposed by "what would fit on a 10-MB system disk."[20][21]

Some of the MicroVMS counterparts differed only slightly from their VMS sibling only slightly, such as using the term $TAPE1: rather than $MUA1:.[22]

Setting software prices was also a problem.[23] Licenses for MicroVAX software had different codes.[24][25] Among the versions were 1.0,[1] 4.2, 4.4,[26] 5.3[27]

Distribution media

"The only distribution medium available was the RX50, a 400-KB floppy disk"[20] until, by version 4.4, when an option for TK50 tapes was added.[28] By VMS 5.0, the MicroVMS/VMS split was discontinued.


  1. (announced October 1983) "MicroVAX". Computer History Wiki (CHWiki). MicroVAX I. Summary. Announcement date: October 1983. FRS date: late 1984. Codename: Seahorse. OS support (VMS):, MicroVMS V1.0
  2. "MicroVAX dictionary definition". line of entry-level VAX computers from Digital Equipment. Introduced in 1983, MicroVAX machines ran under the MicroVMS or ULTRIX operating systems
  3. First Shipped MicroVAX: October 1984
  4. "Loading... Celebrating 30 years of OpenVMS Month Year Caption ..." VAXstation I, code-named "Seahorse," was the first in a new, transitional family of ...
  5. "MicroVAX II". MicroVAX II. Summary. Announcement date: 14 May 1985. FRS date: May 1985. Codename: Mayflower
  6. "The MicroVAX II – Workhorse of the 80's". Microvax2.org (MicroVAX II Museum). Codenamed “Mayflower”, released in 1985, you could have one for $20,000. The MicroVAX II's CPU was the KA630.
  7. Computergram (20 January 1987). "DEC Unveils Real-Time MicroVAX Board, Run-Time VAXELN". Computer Business Review.
  8. Computergram (21 May 1987). "DEC France Does Fault-Tolerant MicroVAX II Configuration". Computer Business Review.
  9. Computergram (11 February 1987). "DEC Puts Old Wine Into New Bottles With MicroVAX 2000". Computer Business Review.
  10. Computergram (29 January 1992). "DEC Launches Three MicroVAXes Designed And Made In Ayr". Computer Business Review.
  11. "DEC Rushes To Rescue Of VAX Users With Four New Models". (23 August 1994). Computer Business Review.
  12. "DEC Upgrades Low-End VAXes To See It Through The Decade". (9 October 1996). Computer Business Review.
  13. Jesse Lipcon. "A letter from Jesse Lipcon".
  14. Computergram (12 April 1994). "DEC Announcements". Computer Business Review.
  15. http://www.vmshobbyist.org/forum/viewthread.php?forum_id=70&thread_id=1310 VS3100 (sic) to Infoserver?
  16. Computergram (20 October 1988). "DEC Aims New MicroVAX 3300, 3400 At AS/400; New Disk Bus". Computer Business Review.
  17. "DEC Hits At AS/400 With MVAX 3800, 3900, Pricing". (13 April 1989). Computer Business Review.
  18. "US Prices For DEC's New MicroVAXes". (11 April 1989). Computer Business Review.
  19. Michael D Duffy (2002). Getting Started with OpenVMS: A Guide for New Users. ISBN 978-0080507354. The result was a new line of computers, ... and a new operating system ... enough to sit beside a desk, running a subset of VAX/VMS called MicroVMS.
  20. Kathleen D. Morse. "The VMS/MicroVMS merge". DEC Professional Magazine. pp. 74–84.
  21. "Micro VMS operating system". Computerworld. June 18, 1984. p. 7. The Micro VMS operating system announced last week by Digital Equipment Corp. for its Microvax I family of microcomputers is a prepackaged version of ...
  22. "MicroVMS 4.7 and VAXStation II". VAXstationII to either a Vaxstation 3100 or VAX 4000. ... The main difference between MicroVMS 4.7 and VAX/VMS 4.7 was a test in the startup file
  23. $5,990 vs. $4,490: a $1,500 difference Paul Polishuk, Editor. "VMS, MicroVMS TCP/IP". Local Area Networks Newsletter. p. 13. The performance of the VAX or MicroVAX host is not affected by TCP/IP protocol ... is $5990 for VAX/VMS systems and $4490 for MicroVAX/MicroVMS systems.CS1 maint: extra text: authors list (link)
  24. H. Rabitz (1987). "Microvax Networked Computer System" (PDF). DTIC.mil.
  25. C. Logg (1988). "A CAMAC and FASTBUS engineering test environment supported by ..." IEEE.org. The system hardware includes a standard MicroVAX II and MicroVAX I with multiple ... The system software components include MicroVMS licenses.
  26. "dec :: vax :: vms :: 4.4 :: AA-HL72A-TN MicroVMS 4.4 ..." bitsavers.org. From the bitsavers.org collection, a scanned-in computer-related document.dec :: vax :: vms :: 4.4 :: AA-HL72A-TN MicroVMS 4.4 Release
  27. "Alsys Limited, Alsycomp 017, Version 5.2, MICRO VAX II Under ..." Alsys Limited, Alsycomp 017, Version 5.2, MICRO VAX II Under Micro VMS V5.3 (Host)
  28. "PDP-8.se". 4.4, AA-HL74A-TN, Installing or Upgrading MicroVMS From a Tape Cartridge, (1) ... 4.4, AA-Z501C-TE, VAX/VMS System Services Reference Manual, (1).
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