Cyrix 486SLC CPU: Introduced April 13,1992

On April 13,  1992, Cyrix debuted its 486SLC CPU. Cyrix didn’t have its own fabrication plants so they relied on other chipmakers, such as SGS Thomson and Texas Instruments, to manufacture the chips. Part of the agreement allowed TI to make its own derivatives of the chips, and share the advances back to Cyrix. The 486SLC was really more a 386SX/486SX hybrid than a true Intel 486 clone. It plugged into a 386SX socket and had the 486 instruction set and 1K of L1 cache. But clock for clock the Intel 486 was faster in a fair fight, and having just a 16-bit external bus kept it from being a fair fight.

The Cyrix 486SLC CPU

Released about three years after Intel’s 486, the Cyrix 486SLC could run at up to 40 MHz. But a 486SLC was closer clock for clock to a 386SX in performance than to a 486SX in performance, because it used the same 16-bit external bus as the Intel 386SX and had a smaller cache. The 486SLC allowed motherboard makers to extend the lifespan of their 386SX boards, letting them sell them as 486 boards with a BIOS upgrade, and with the ability to take a 387sx math coprocessor. They could also extend the life of their 386SX upgrade products that upgraded earlier CPUs like the 286, giving a proprietary 286-based system that couldn’t take a motherboard swap another upgrade path.

For applications that demanded better performance, Cyrix also offered its 486DLC CPU, which had a 32-bit bus.

A 486SLC with a math coprocessor still wasn’t as fast as a 25 MHz 486DX. But it could then outperform a 486SX at math operations while being cheaper than an Intel DX.

It wasn’t just the smaller L1 cache that limited its performance. The slower 386 bus also slowed it down, and the Cyrix core was about 10 percent slower than the Intel 486 core, partly because Cyrix didn’t implement the Intel 486’s burst transfers.

Magazines at the time insisted the Cyrix was a 486-class chip because it did contain all of the 486 instructions and could run any software written for the Intel 486. But it was very much a budget chip at the low end of the market.

Why Cyrix only gave you 1K of L1 cache

If you wonder why Cyrix only put 1K of cache on its 486SLC and 486DLC chips, it was probably cost. Cyrix’s profit margins were pretty bad because they were paying a premium to use someone else’s fabrication plants, so cutting the cache to 1K kept the transistor count lower and the chip size smaller. Bigger caches tend to be faster, but as cache size increases, the return on investment decreases. The first 1K makes the biggest difference.

In its Nov 15, 1993 issue, Microprocessor Report estimated that building the Cyrix-derived chips with 8K of L2 cost TI $37 per chip to make in late 1993. At that price, selling the chip for $119 wasn’t possible. But if Cyrix could get the cost of production plus what it owed TI down to $40 or less, they could sell for $119 and do OK. But meanwhile, Intel was churning out 486 CPUs for $19 apiece and getting much more than $119 a pop for them.

Texas Instruments 486SXLC vs Cyrix 486SLC

TI’s 486SXLC, a 16-bit version of the 486SXL, is interesting because TI went ahead and implemented an on-chip 8K level 1 cache like Intel, and while that improves performance over a 386SX or a Cyrix 486SLC, a TI486SXLC still isn’t as fast as an IBM 486SLC2 running at the same clock rate. TI also added clock doubling, so you could run the chip on a 1X multiplier if you had a 33 or 40 MHz bus, or 2x multiplier if you had a 20 or 25 MHz bus to reach 40 or 50 MHz.

This is largely because the TI 486SXLC lacked the burst transfers of the regular 486, which imposed about a 10 percent performance penalty even when all else was equal.

You can also find 486SLC chips with TI branding on them. These are the same as a Cyrix 486SLC, right down to the 1K of L1 cache.

IBM 486SLC2 vs Cyrix 486SLC

IBM sold a similarly named CPU called the 486SLC2. This was confusing because both the IBM and Cyrix chips appeared on the market within a few months of each other. But they were completely unrelated designs. IBM also put a 486 core in 386SX packaging and added cache, but IBM added 16K of L1 cache versus the 1K of cache Cyrix used. The IBM design was also based on Intel’s 486 core, and the chips bore an Intel copyright because of it. IBM had the rights to manufacture Intel 386 and 486 CPUs, and also to create its own derivatives from the designs.

IBM’s 486SLC2 still had to live with the limitations of the 386SX bus, but the generous L1 cache and more efficient Intel-derived core compensated somewhat.

Confusingly, Cyrix did partner with IBM a few years later, during the 6×86 era, to provide manufacturing. But the 486SLC and 486SLC2 predate this relationship. IBM used the SLC designation first but didn’t trademark it, so nothing stopped Cyrix from using it.

The 486SLC’s legacy

The 486SLC put Cyrix on the map as a scrappy maker of value-priced CPUs. The Cyrix 5×86 of a few years later was the spiritual successor to the 486SLC, putting a Pentium-like core in 486 packaging. Its most successful CPU was the 6×86, a Socket 7 chip that was faster clock for clock than a Pentium, at least for integer operations. It beat AMD to market, so the 6×86 sold fairly well until AMD released its K6. The 6×86 was overmatched against the K6.

Enabling the cache on a Cyrix CPU

Paul Gortmaker wrote a utility for Cyrix CPUs called cyrix.exe that works to enable the 486SLC’s cache if your motherboard doesn’t. This post on Vogons details its use at the very top. Scroll to the end of the first message in the thread to download an archive that contains the cyrix.exe utility. Ardent Tool also has a page containing links to the utility.

The command cyrix.exe -i1 -f will generally get the Cyrix 486SLC’s cache running on most motherboards and give you the speed increase over a 386SX that you’re looking for.

Dave Farquhar

David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.