OS/2 Museum

While researching the history of 486s for a previous article, I came across a fascinating Wikipedia entry and its associated talk page. It’s a nice showcase of inmates running the asylum, and a reminder that Wikipedia can’t be considered an authoritative source of information because the quality of encyclopedia entries is wildly uneven. It’s not all bad, and it could be argued that the average is quite good, but there’s no minimum quality guarantee.

Even now the 486SX article contains demonstrably incorrect claims such as this: The FPU upgrade device was shipped as the i487, which was a full blown i486DX chip with an extra pin. The i487 was installed in an upgrade socket and the extra pin was either a power or ground pin that indicated that the i487 was installed. That signal was used to disable the i486SX when the i487 was installed. 

Well, no—there is an extra pin, but it has no electrical function whatsoever. It merely prevents the chip from being installed incorrectly. And there is a pin (NC#) indicating that an upgrade processor is present designed to “shut off” the original CPU, but it is one of the standard 168 pins. The text conflates facts and turns them into a falsehood. The pins are publicly documented in Intel datasheets and it’s not so hard to find the actual information. Continue reading →

Intel had a long history of offering retail processor upgrades for PCs. The last and by far the best known of those were the Intel OverDrive processors. But let’s start with the earlier history.

In 1987, Intel released the Inboard 386/AT, an ISA card with a 386 CPU used to upgrade existing PC/AT systems to 386s. In 1988 the Inboard 386/PC followed, a similar card for upgrading 8086-based PCs.

In 1992, Intel offered RapidCAD, a two-chip (for the CPU and FPU sockets) upgrade for 386 systems. The RapidCAD used a somewhat crippled 486 core with integrated FPU and although it didn’t greatly improve integer performance, it did significantly increase the floating-point power (70% improvement according to Intel) to overtake any discrete 387.

i486 DX, SX, and 487SX

When Intel introduced the i486 in 1989, the benefits (for Intel) of segmenting the market with SX and DX variants were well understood. Whereas the 386DX (1985) used a 32-bit external interface and the 386SX (1988) a 16-bit one, the 486DX and 486SX were almost identical in terms of external interface. The DX variant included a built-in FPU, the SX did not.

Early i486SX Processor

The 486SX was introduced in mid-1991, and its main purpose was to compete with faster 386 systems, especially those from AMD. At that time, the vast majority of software did not require a FPU, and most software didn’t use one even if available. Although the 486 had considerably better performance per clock, a 40 MHz 386 compared quite favorably to a much more expensive 25 MHz 486. The 486SX was intended to get OEMs to roll out 486 CPUs (which only Intel produced at the time) in larger numbers, understandably at prices lower than the top-of-the-line DX variants. Continue reading →

There don’t seem to be any specification updates or errata lists for any Intel 486 CPU anywhere. It’s odd because there are specification updates for 386s (and of course Pentiums) from Intel, and because the embedded 486s continued to be produced for a long long time (standard 486s well into the 2000s).

The 486 was released back when Intel believed that publishing errata would be damaging and it seems the policy was never changed, as it had been for the Pentium (after the infamous FDIV fiasco).

Someone, somewhere has the errata sheets. Anyone want to share? Continue reading →