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As I was skimming through an old MS-DOS game's README, I stumbled upon this:

Therefore, we reccomend a newer 486-100 or better, preferably 
with a large external cache. Best performance will result 
from play on a 486DX3/4 75/100 or Pentium system with
interleaved (72 Pin SIMM, or EDO) memory.

Wait, 486 DX what? DX3? Heard of DX, DX2 and DX4 but never heard of DX3!

Searching online shows a few results but no definitive answer:

Info: 486DX3 from IBM:

IBM has recently announced the 486DX3 clock trippling processor.

Intel CPUs:

The 2.5X mode was originally meant to be used in a 486DX3-63,83 (2.5X clock) but these were deemed to be unmarketable.

Where did 83 MHz come from, and why didn't it catch on?

Also, I believe the reason there is a 486DX4-100 rather than 486DX3-100 is that there was supposed to be a 486DX3-83.

Question:

Urban legend or could you really buy a 486DX3 back then?

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  • 5
    Not enough for an answer, but the DX3, running at 2.5x rate to the bus clock, was never released. The DX4, running at 3x rate to the bus clock was released.
    – Justme
    Commented May 10 at 6:49
  • 1
    Could be a typo. Commented May 10 at 14:54

1 Answer 1

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The short answer is no, there never was a DX3 part.

486 CPUs from Intel have a varied naming history, mostly because of trademark concerns: Intel wanted to stop its competitors selling CPUs with similar names to its own. Perhaps relatedly, CPU manufacturers competing in the x86 world in those days tended to talk about forthcoming developments before naming them; as a result, magazine journalists sometimes used unofficial terms — see this PC Magazine “Clock Triplers” article for example, or this John C. Dvorak column (as always, take such columns with a large pinch of salt).

So people were talking about DX3 CPUs before they existed. When Intel’s clock-tripled 486s (which weren’t the first clock-tripled x86 CPUs¹) were released, they were branded IntelDX4. It’s not entirely clear why DX3 was skipped — there are claims that it’s related to trademark issues and a dispute with AMD, but the US PTO doesn’t have any information on “DX3” CPU branding requests, nor does it have any for “DX4”, and the only related trademark that existed for any length of time is i486 (in two variants)².

Intel did talk about a 2.5× 83MHz part, but when that was referred to with an Intel brand name, it was referred to as DX4 too. That was never released. Released Intel DX4 CPUs can use a 2× or 3× internal clock multiplier; this is controlled using CLKMUL on pin R17. It’s possible that this is a tri-state pin also allowing for 2.5× but I haven’t been able to confirm that.

It’s possible that DX4 refers to the manufacturing process rather than any specific clock multiplier; the DX4 CPUs used a 0.6µm process, smaller than previous 486 CPUs.

(Bear in mind that game README files and magazine articles are written by sometimes ill-informed people, through no fault of their own; magazine journalists in particular in the 90s sometimes repeated what they were told by manufacturers, uncritically, because that was all they could really do.)


¹ IIRC, the first was IBM’s Blue Lightning.

² Intel tried to register 486 and Intel486, but they were rejected or invalidated.

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  • The rumors about the DX3 at 2.5x clock frequency include that CLKMUL is a three-way input pin, see for example vogons.org/viewtopic.php?p=740561#p740561 . Other people claim CLKMUL=Vcc is 2.5x. Commented May 10 at 19:09
  • Yes, that would be a possibility — I’m looking for my DX4 hardware manual to check whether it’s mentioned there. My point with that is really that “DX4” didn’t seem to be specifically tied to 3× (beyond the fact that all DX4 CPUs which actually made it to market defaulted to 3×). Commented May 10 at 19:32
  • My guess: they wanted to use DX4 to block AMD's use of the same.
    – user71659
    Commented May 11 at 17:57
  • @user71659 that fits in with their general strategy (that led in particular to the Pentium name), but I would expect to at least find a trademark filing for “DX4” and/or “IntelDX4”... Commented May 11 at 18:23
  • @StephenKitt You don't have to register a trademark to get rights in it. If the situation is uncertain, as it was with "486," it may be preferable to leave it uncertain than for USPTO to say no.
    – user71659
    Commented May 12 at 17:51

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