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The DEC DECstation 2100 / DECstation 3100 Page

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not endorsed by (the former) Digital Equipment Corporation

(or Compaq, or even HP, for that matter since they've subsequently been bought and bought out again...) Anyway, if you see any corrections or additions that would make this web page better, please feel free to email the author.

Picture of Diablo, my DECstation 3100 PMAX

Official page URL:

digital (Compaq^H^H^H^H^H^H HP...) no longer builds machines of this line, as they have moved over to their own cpu, the AXP21x64(tm) series, or better known as the Alpha. Ok now it seems that now Alpha is being dropped. And time to move on to PA-RISC.

Well, not even that, now make another change, let's all go to the Itanium Processor Family. Lots of change going in. Anyway, DEC/Compaq/HP has completely dropped support of Ultrix, the DECstation's supplied operating system, as of 1999. We will all have to move over to NetBSD, OpenBSD, or Linux when it is completed. Currently Linux is probably the worst supported because of hardware driver lackings. I've personally used NetBSD and it works fine. Oh. OpenBSD looks like they've dropped pmax support so you'll need to use an old version. So it looks like NetBSD and nothing else... and oh wait, NetBSD is "dying"... Well, despite all the hubbab about NetBSD, it's coming out with release 7 sometime soon and as of 6.1.5 *still* supporting the DECstation where other OS tends to drop support on a machine made almost 3 decades ago!

The Digital DECstation 2100/3100 is one of the first commercially available RISC-Architecture based machines, released in 1989. It's Digital's first RISC machine after the venerable PDP-series minicomputers and VAX-series micros.

The DECstations are known as the PMAX series, based off of the three chip MIPS R2000/R2010/R2020 CPU set, and runs at 12 or 16.67 (for the 2100 or 3100, respectively) MHz. The technical code name for the 2100 is PMIN but it's the same machine at 12MHz instead of the faster 16MHz PMAX and have possibly slower cache chips and thus was cheaper due to the lower-binned parts. Both use the same layout KN01 mainboard. I will refer to both as PMAX on this webpage and indicate clock speed when there is a critical difference between the two (a PMAX running at 12MHz == PMIN).

Its case formfactor is a medium profile desktop unit, and was designed with two 3.5" disk bays and one 5.25" half height bay (1.6") which can be used for removable storage. The right hand bay was designed for a 3.5" SCSI floppy drive and the hole in the metal shield isn't large enough for a cdrom. The other 3.5" bays need special trays that are screwed onto the platform. My DECstation does not have any of the auxillary trays nor a floppy drive, so I grabbed one of those PC 5.25 to 3.5" adaptors and put an internal disk into the floppy bay. In any case, it has a built-in standard SCSI-1 controller using the custom SII interface chip, though the rear external connector is nonstandard. See link in the pinouts section for details.

The MIPS R2000 is a scalar, RISC (uniform instruction length) 5-stage pipelined processor with static taken branch prediction (and that cursed delayed branch that all MIPS programmers know all too well). It has separate direct mapped, 16K entry/16 bit tag, 64KB D- and I-caches which run at full speed. They're write through caches but use the R2020 to buffer writes so the processor doesn't need to stall as often without having to implement write-back. The 16MHz PMAX can sustain about 11 MIPS, which is faster than a 386DX/40 and is about the same speed as a 486DX/33 which also has static branch prediction. (At least the Dhrystone benchmark reports the DS3100 as being 50% faster than my 386DX/40 and my 486DX/33 was about 50-60% faster than the 386/40.

Keep in mind we were still dealing with mostly 16-20 MHz 386's in 1989 so this is considerably faster. Though the 386 was also pipelined, branches (386's flush upon branch regardless), load/stores (x86's have to do a lot of them due to the limited architected register file size), and floating point (Intel's weird x86 stack based FP architecture from the 8086/8087 days -and- the fact the 387 coprocessor was an option for most 386 machines) makes them notoriously slow. This put a big performance penalty on the Intel processor. Thus, the R2000, even at a lower clock speed, simply blows the Intel 386 away. In fact, in 1989, the DECstation 3100 topped in the Dhrystone benchmark, beating the SparcStation 1 by a bit, and blasting the 386DX/33 away. As always, the crown is short lived when the SparcStation 2 was released.

By today's standards, the DECstation 3100 is extremely slow. My i7 runs at least a thousand times faster, LITERALLY. Not even exaggerating one bit here with the three orders of magnitude. The MIPS Emulator is also now significantly faster than the machine. Distcc would help too, alas the meager 24MB RAM in the DECstation limits usability ...

Some useful references

  • PMAX Specs sheet
  • PMAX Boot ROM/POST information
  • DECstation interface Pinouts
  • KN01 board with notes
  • DS3100 Functional Description v1.3, 28 Aug 1990 (converted to portable document format)(192K bytes)

  • Thanks to Kirk Russel for scanning these! Remove spam and change dot to fix. His webpage about DECstations is at
  • PMAX Hardware Installation Guide and Local Mirror (1.4M bytes)
  • PMAX Operators Guide and Local Mirror (2.5M bytes)

  • DEC Harddrives
  • DECstation 5000 (under construction)
  • (bad link)PMAX Boards type list
  • Software Issues
  • digital Corporation (Now HP)

  • About my DECstation 3100 "Diablo"
  • Internal PMAX Pictures
  • DECstation Wikipedia page
  • NetBSD
  • MIPS and DECstation Emulator or (orig) (Doesn't totally emulate all DECstation 3100 PROM calls quite yet..., but will emulate a DECstation 5xxx
  • DECstation PMAX/PMIN Discussion Board
  • More DECstation Linux pages
  • Copyright 1997-2005, Remove the SPAM before replying please! Anyway, this is my humble and slowly hacked Home page?. I appreciate any comments and corrections. Information may not be correct and is not guaranteed. Excerpts/information taken from the DECstation 3100 Hardware/Installation/Operator's Guide and the Functional Description Guide, (c)Digital Eq. Corp.
    This page is advertising banners free from the owner's private network connection. All I ask is that you consider using Hewlett Packard servers based on Intel Itanium Processor Family if you find this page useful.

    June 2005, included InstGuide and OpGuide
    August 2005, fixed a spelling typo and clarified wording near that section
    September 2008, new domain name