UMC Green CPU: The forbidden 486

UMC Green CPU: The forbidden 486

Dave Farquhar Retro Computing June 5, 2024

Last Updated on September 16, 2024 by Dave Farquhar

Clock for clock, the best 486 was a CPU you might not have been able to buy and may very well have never heard of. Aside from Intel, AMD, and Cyrix, there was a fourth 486 family that didn’t get very far due to legal issues: the UMC Green 486 CPU. In this blog post, we will cover what made this 486 so special and what made this forbidden 486 illegal.

In the mid 90s, I built a lot of 486 PCs for friends. I built at least one or two with an Intel CPU for the purists, but I usually used AMD CPUs because of their price ratio. An Intel DX4-100 ran faster than an AMD DX4-100, but the AMD DX4-100 cost about the same as an Intel DX2-66. And when someone was on a really tight budget, there was Cyrix.

And for a little while, the motherboard manuals mentioned a fourth CPU manufacturer, UMC. Of course I was curious about them, but because I was in the middle of the United States, I never saw one for sale. And I never read about it in any of the computer magazines I had at the time.

Then the prices on Pentium-class CPUs dropped and I forgot all about that mysterious UMC 486 for about a quarter century. And that is kind of a shame because the UMC 486 really was my kind of CPU.

Who is UMC?

UMC, or United Microelectronics Corporation, is a Taiwanese chip manufacturer who has their own fabrication plants. Established in 1980, they were the first private integrated circuit company in Taiwan. In the 1990s, they were one of the major manufacturers of chipsets for the 486. But they have been making computer chips for a long time. They were a second source for the 6502 CPU and its supporting chips, for example. Atari was one of its customers for those chips.

Much like AMD and Cyrix, UMC was well aware of Intel’s profit margins on their CPUs, and they wanted a piece of that market. They knew they could undercut Intel’s price while still leaving a healthy profit margin for themselves.

UMC’s 486 vs Cyrix

UMC had an advantage over Cyrix, because they had their own fabrication plant. Cyrix had to outsource fabrication to other companies like Texas Instruments, ST Microelectronics, and IBM. This increased overhead, but it also meant Cyrix didn’t always have the manufacturing capacity they needed to keep up with demand.

Cyrix never had access to Intel’s microcode, so they had to build their own implementation. And theirs wasn’t as fast. Typically, when you run a benchmark, a Cyrix runs 20 to 25% slower than a comparable Intel or AMD. The performance difference wasn’t ruinous, and Cyrix tried to make it up to you with very aggressive pricing. It also means unless you have an emotional attachment to Cyrix, there isn’t much reason to use a Cyrix 486 in a retro build. That’s because the Intel version is more common, it’s usually cheaper to buy today, and it’s faster.

UMC had the same problem as Cyrix, so they had to implement their own microcode. But while the Cyrix implementation was less efficient than Intel, UMC did the same thing NEC had done with the V20, making a compatible implementation that was more efficient clock for clock. UMC was 30% faster at integer division than Intel, for example. The efficiencies weren’t linear, but generally speaking, when Cyrix was punching a speed grade below its weight, UMC was generally punching a grade higher.

UMC’s 486 vs AMD

AMD had its own fabrication plants, and they also had access to Intel’s microcode. AMD had been an authorized second source for Intel x86 CPUs up until the 286 days. After Intel cut them off, AMD was able to use its existing relationship to maintain access to Intel’s 386 and 486 microcode. The result was that an AMD 486 performed almost identically to an Intel 486 as long as the clock speed and the cache matched up. It did turn into something of a cat and mouse game, with Intel implementing a writeback cache and then increasing the cache size to 16 KB, but when the specs lined up, they performed about the same.

UMC outperformed Intel at the same clock speed. Because of that, UMC also outperformed AMD.

What was the catch with the UMC Green 486 CPU?

There was a catch. First, the UMC Green 486 CPUs were SX chips, not DX. UMC developed a DX2 variant but never put it into production. Secondly, they were available at speeds of 25, 33, and 40 megahertz. So you couldn’t get the full range of speeds Intel, AMD, or Cyrix offered.

But the performance at the released speeds made for a very impressive debut. The 40 megahertz version kept pace with the elusive 50 MHz Intel DX, and even the 66 MHz DX2, as long as you weren’t doing floating point math.

The UMC Green 486 CPUs tend to overclock rather well, so that allowed hobbyists to project its speeds out even higher. It is not as fast or efficient as a Pentium at the same speed, but it was somewhere between the two in performance while being less expensive than a 486.

It would have been the best value going.

What went wrong for UMC?

The UMC Green 486 CPU violated Intel patents for memory management and extended memory addressing. And UMC didn’t have a patent sharing agreement with Intel. Furthermore, Intel had no motivation to negotiate such an agreement. Instead, they sued in 1994 and 1995 in various venues. The two parties settled in 1996, with UMC agreeing to withdraw its chips.

UMC Green 486 CPUs have a marking on them saying not for sale in USA. I’ve heard some speculation that this was part of an agreement between the two companies. But seeing as the UMC chips never were sold in the United States, it seems UMC was hoping they could avoid issues by not selling in the United States.

That didn’t work out, so Intel was able to force the UMC 486 off the market.

UMC’s 486 is a great might-have-been. It’s mostly interesting to CPU collectors, rather than as a practical CPU to power a hobby machine. It’s fun and interesting nevertheless, especially for people who like forgotten, underrated and underdog technology.

UMC wasn’t the first chipset maker with this idea. Just a few years before, fellow chipset maker Chips & Technologies ventured into this field with its Super 386. The results were eerily similar.