I’m currently building an 80386-compatible core in SystemVerilog, driven by the original Intel microcode extracted from real 386 silicon. Real mode is now operational in simulation, with more than 10,000 single-instruction test cases passing successfully, and work on protected-mode features is in progress. In the course of this work, corners of the 386 microcode and silicon have been examined in detail; this series documents the resulting findings.

In the previous post, we looked at multiplication and division -- iterative algorithms that process one bit per cycle. Shifts and rotates are a different story: the 386 has a dedicated barrel shifter that completes an arbitrary multi-bit shift in a single cycle. What's interesting is how the microcode makes one piece of hardware serve all shift and rotate variants -- and how the complex rotate-through-carry instructions are handled.

When Intel released the 80386 in October 1985, it marked a watershed moment for personal computing. The 386 was the first 32-bit x86 processor, increasing the register width from 16 to 32 bits and vastly expanding the address space compared to its predecessors. This wasn't just an incremental upgrade—it was the foundation that would carry the PC architecture for decades to come.