80386 Barrel Shifter
owl writes:
https://nand2mario.github.io/posts/2026/80386_barrel_shifter/
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.
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