z386: An Open-Source 80386 Built Around Original Microcode

This is the fifth installment of the 80386 series. The FPGA CPU is now far enough along to run real software, and this post is about how it works. z386 is a 386-class CPU built around the original Intel microcode, in the same spirit as z8086. The core is not an instruction-by-instruction emulator in RTL. The goal is to recreate enough of the original machine that the recovered 386 control ROM can drive it. Today z386 boots DOS 6 and DOS 7, runs protected-mode programs like DOS/4GW and DOS/32A, and plays games like Doom and Cannon Fodder. Here are some rough numbers against ao486: Metric z386 ao486 Lines of code (cloc) 8K 17.6K ALUTs 18K 21K Registers 5K 6.5K BRAM 116K 131K FPGA clock 85MHz 90MHz 3DBench FPS 34 43 Doom (original) FPS, max details 16.5 21.0 In current builds, z386 performs like a fast (~70MHz) cached 386-class machine, or a low-end 486. It runs at a much higher clock than historical 386 CPUs, but with somewhat worse CPI (cycles per instruction). The current cache is a…