Early Problems and Upgrades

Early Problems

Intel originally intended for the i386 to debut at 16MHz, but due to poor yields was instead introduced at 12MHz.

Early in production, Intel found a bug that could cause a system to unexpectedly halt when running 32-bit software. Not all of the processors already manufactured were affected, so Intel tested its inventory. Processors that were found to be bug-free were marked with a double-sigma (ΣΣ), whilst affected processors were marked "16 BIT S/W ONLY" and sold as otherwise-good parts, since at the time 32 bit software was not relevant for most users. Such chips are now extremely rare.

The i387 Math-Coprocessor was not ready in time for the introduction of the i386, and so many of the early 386 motherboards instead provided sockets to make use of an 80287. In this configuration the FPU would operate asynchronously to the CPU, usually with a clock rate of 10MHz. The original Compaq Deskpro 386 is an example of such design. However, this was an annoyance to those who depended on FPUs as the performance of the 287 was nowhere near that of the 387.

Upgrades

Intel offered a modified version of its later 80486DX in 80386 packaging, branded as the Intel RapidCAD, to provide an upgrade path for 80386 users. The upgrade worked as a pair of chips that replaced both the 386 and 387, although since the 80486DX contained an FPU itself the chip that replaced the 387 served no purpose except to appear like a coprocessor, so that the system board would be configured correctly. The CAD branding can be explained by the massive increase in floating point performance offered; integer performance increase was around 30%.

A wide range of 3rd party upgrades were also available for both SX and DX systems. The most popular ones were based on the Cyrix 486DLC/SLC core, which typically offered a substantial speed improvement via more efficient instruction pipeline and 1kb (or sometimes 8kb in the TI variant) internal L1 SRAM cache. Some of these upgrade chips (such as the 486DRx2/SRx2) were marketed by Cyrix themselves, but were more commonly found in kits offered by upgrade specialists such as Kingston, Evergreen and Improve-It Technologies. Some of the fastest CPU upgrade modules featured IBM SLC/DLC family (notable for its 16kb L1 cache), or even the Intel 486 itself. Many 386 upgrade kits were advertised as being simple drop-in replacements, but often required complicated software to control the cache and/or clock doubling. Overall it was very difficult to configure upgrades to produce the results advertised on the packaging and often less than 100% stable/compatible.