IBM 486SLC2 CPU: When a clone isn’t a clone

Last Updated on March 15, 2023 by Dave Farquhar

The 486 SLC2 is an oddball 486 chip made by IBM. It isn’t something anyone seems to talk about much, maybe because I’m the only one who finds its story ironic. It’s the story of a proprietary upgrade that found second life on clone motherboards. Except the clone motherboards were made by IBM themselves. If IBM made it, is it still a clone?

Initially IBM released the 486 SLC2 in the summer of 1992. It was an upgrade for the 386 SX-based PS2/s. IBM later released PS2s that came with this chip from the factory. For what it’s worth, I used and worked on these systems extensively when I was in college.

But eventually, IBM started manufacturing AT form factor motherboards with ISA slots and an Award BIOS that fit in any clone case. Some of the boards even had a statement on them that they were made by IBM. And they were cheap. When I was working at Best Buy, someone came in and was bragging about how he built systems using those boards and they were cheaper than anything we sold. If you couldn’t afford a Packard Bell, you could buy… IBM instead?

What the 486SLC2 was

This clone motherboard was actually made by IBM, so IBM could sell its 486SLC2 CPU. Intel wouldn’t let IBM sell the chip alone, but they could sell motherboards if they wanted.

If you are familiar with Cyrix, that SLC name may sound familiar. And IBM sometimes made chips for Cyrix. For that matter, as part of the agreement, IBM sold half of the yield under its own name. But that’s not where the 486SLC2 came from.

IBM also had an agreement to make chips based on Intel’s designs. This arrangement ended with the Pentium generation, and it was IBM who ended it, but IBM did make 486 chips based on Intel’s technology. You can find conventional Socket 3 Intel 486 CPUs with Intel branding and an annotation on them saying they were made by IBM. And sometimes they did strange mashups with that technology. That’s what the SLC2 was. IBM took the 386SX chip, put a 486 core in it, clock doubled it, and doubled the cache to 16k to make up for the 16-bit data bus. The result was what everyone assumed the 486SX was but wasn’t. Performance wise, it was a little bit better than a 33 megahertz 486SX, and it was cheap to make. The “SLC” stood for “super little chip.”

IBM made an upgrade board with this chimera chip on it, priced it at $259, then quickly raised the price above $900. Because IBM.

Critical reception of the IBM 486SLC2

When it was first released, Infoworld magazine gave it a mixed review in its July 20th, 1992 issue. It was a nice upgrade from a 25 MHz 386SX, and they didn’t object too loudly to the $259 price, but they lamented it only worked in IBM machines.

IBM later released an upgrade module based on the chip for 286 systems. They didn’t sell it themselves, but manufactured it and sold it through Kingston, the famous maker of memory modules. Of course, on most 286 systems you could just swap a motherboard to get a 486 upgrade, but the IBM module worked on PS/2 machines, which were designed not to be able to take a motherboard swap. Not a commodity board swap, at least.

IBM 486SLC2 motherboards for clones

This ad shows how selling the IBM 486SLC2 put IBM in the position of competing with itself. The SLC2 was a viable budget CPU for a couple of years.

The motherboards started appearing sometime in 1993. Various no-name clone makers advertising in the back pages of computer magazines started touting inexpensive systems with IBM technology inside them. They looked like clones, any other run of the mill clones, but the ad promised genuine IBM quality and compatibility. And even CompuAdd, a fairly well known brand at the time, sold SLC2-based systems.

The motherboards themselves eventually turned up for separate sale. I found an ad from 1995 advertising one of the boards alongside several others. A 40 megahertz 386DX cost $245. An IBM 486SLC2 running at 66 megahertz cost $295. A more conventional motherboard with an AMD 486 processor cost $360 for 40 megahertz and $410 for 80 megahertz.

The IBM 486SLC2 wasn’t a great performer, because of that 16-bit bus. It could only take a maximum of 16 MB of RAM, which was a liability by 1995. It was pretty common knowledge by then that Windows 95 ran reasonably well with 16 MB, but 24 or 32 MB was better. An SLC2 board wasn’t very future proof.

Why IBM made motherboards for competitors

But it still seems odd that IBM would make motherboards for cut-rate clones. IBM wanted to sell the chips, but Intel didn’t allow them to sell the chips alone. But they could sell them if they were on a circuit board. Hence the upgrade boards IBM made for Kingston. And the agreement also allowed them to make motherboards. The March 22, 1993 issue of Infoworld explained this arrangement.

It was an odd about-face for IBM. The whole idea of the PS/2 line was to put an end to cloning. But by 1993, it was clear that battle was over and IBM lost. So IBM decided to try to profit off it. IBM sold hard drives to clone makers too, so making motherboards for them wasn’t too much of a stretch.

Who bought these IBM non-clone clone boards

The appeal was really for people who needed a cheap system and ideally didn’t need to run Windows 95 on it. Windows 95 was immensely popular, but just like any new operating system, it had a lot of holdouts. There were some people who were perfectly happy to stick with Windows 3.1 or even just DOS. And these relatively inexpensive IBM-made boards were just fine for those types of uses.

But they weren’t mainstream. If you didn’t peruse the ads in the back pages of computer magazines, you probably weren’t aware of them. They were generally advertised by small resellers that didn’t have a lot of ad budget. Intel and AMD 486s were much more typical mainstream choices, and Cyrix 486s were a more common budget choice. Any of them were better for Windows 95.

I built a couple of systems based around these boards in the mid-90s. In one instance I even put one in an IBM XT case. It said IBM on the outside, and when you fired it up, it said IBM when you booted it. It was a fun sleeper system.

If you have a run across a clone board in an XT that claims to be a 486 and claims to be made by IBM, it’s probably not lying. It’s a historical oddity. And maybe it’s the one that I cobbled together way back when. In theory one of these boards would have fit in a 5150 case, with the caveat the maximum five slots would be usable and the spacing may have precluded even that many. But 5150 cases were extremely cheap in the ’90s because not many people were aware you could even do that with them.

VLB and the IBM 486SLC2

Adrian Black recorded a video about what he thought was a 386SX board with VLB slots. In all likelihood, his board was an IBM 486SLC2 board of some sort with VLB slots. He expressed surprise that anyone would pair up a 32-bit local bus with a CPU that only had a 16-bit data bus, and wondered aloud what the point was.

He’s right that the CPU’s bus limited the benefit. But the math suggests it was still worth something. The VESA slots were running at 33 MHz, the same as the CPU’s front side bus. The overhead of the CPU cut the effective speed in half, but the effective speed was still double that of 8 MHz ISA, besides the benefit of the graphics chip on a VLB card usually being more powerful than what you’d generally find on an ISA video card.

Of course, if a board had a 50 MHz 486SLC2, the benefit was more limited, since the effective speed of the local bus was 12.5 MHz. Still better than ISA, but I’d be more inclined to question whether it was worth the premium of a VESA video card and IDE adapter.

At least one of the 66 MHz 486SLC2s I built in the 90s had VLB. And while I never benchmarked it against a 386DX-40 system that was all ISA, it was a better Windows 3.1 machine than the 386DX-40 I had during the same timeframe.

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