Keep in mind, that this was Intel's flagship processor, From October 1985, until April of 1998, and they had tried to eliminate all the second sourcing. It wasn't until 1989, that the Am386 was released, and out came all the lawyers.
They were using the 6th and 7th bytes of the GDT/LDT, which were reserved, and since it affected protected mode, and virtual mode addressing, was likely stored in the microcode. Which affected Xenix, and pissed off Intel enough, that they fixed their version of Xenix, and no one else's, SCO did a rewrite and charged $500 for the privilege of running a multi-user OS.
Add to #8, the new addressing modes, the new protected modes, which affected ALU OPS, Moves, calls, redirects, and indirects.
#7, the Microcode entry points are linked directly to the instruction decode logic, and of course not limited to the great LOADALL instruction, and the new multi-stage instruction pipeline, and prefetch.
This took years for AMD to blackbox the 386, and then:
"1987–1992: The arbitration proceeding, originally expected to take only six weeks, dragged on for nearly five years."
"The 80386 microcode was successfully extracted and publicly disassembled by a team of hardware historians and demoscene researchers (including reenigne, Ken Shirriff, and others). They extracted the 94,720-bit microcode ROM from 80386 die shots by combining image processing, neural networks, and human-aided automation. AI tools played a crucial role in cleaning up the die images, detecting cell patterns, and binarizing the data before humans parsed the 37-bit microinstruction formats. You can read about the full process on the Reenigne Blog.By contrast, the 8086 microcode was extracted through purely human-driven analysis of die photos. The 8086's 21-bit microcode is simpler and was fully reverse-engineered and disassembled in 2020. You can explore the decoded 8086 microinstructions interactively using the nand2mario 8086 Microcode Browser.8086 Microcode Browser - Small Things Retro - nand2marioDec 4, 2025 — Since releasing 486Tang, I've been working on recreating the 8086 with a design that stays as faithful as possible to the original...GitHub80386 Microcode Disassembled - Reenigne blogMay 23, 2026 — Well, they may have taken that as a bit of a challenge - they threw various bits of image processing, neural networks, and human-a...www.reenigne.org80386 microcode disassembled « Reenigne blog - daily.devMay 23, 2026 — 80386 microcode disassembled « Reenigne blog. A detailed account of disassembling the Intel 80386 microcode ROM, a 94720-bit blob ...daily.devi386 - WikipediaMicrocode reverse engineering In May 2026, the Intel 80386 microcode was reverse engineered and publicly disassembled by a group i...Wikipedia8086 microcode disassembled - Reenigne blogSep 3, 2020 — Recently I realised that, as part of his 8086 reverse-engineering series, Ken Shirriff had posted online a high resolution photogr...www.reenigne.orgThe 386 microcode has been fully reverse engineered - Reddit May 24, 2026 — In a group effort, a bunch of demoscene legends like reenigne have reverse engineered the microcode for the 80386, opening the pat...Reddit·r/thisweekinretro80386 microcode disassembled « Reenigne blog | daily.devMay 23, 2026 — 80386 microcode disassembled « Reenigne blog. A detailed account of disassembling the Intel 80386 microcode ROM, a 94720-bit blob ...daily.dev"
They were using the 6th and 7th bytes of the GDT/LDT, which were reserved, and since it affected protected mode, and virtual mode addressing, was likely stored in the microcode. Which affected Xenix, and pissed off Intel enough, that they fixed their version of Xenix, and no one else's, SCO did a rewrite and charged $500 for the privilege of running a multi-user OS.
Add to #8, the new addressing modes, the new protected modes, which affected ALU OPS, Moves, calls, redirects, and indirects.
#7, the Microcode entry points are linked directly to the instruction decode logic, and of course not limited to the great LOADALL instruction, and the new multi-stage instruction pipeline, and prefetch.
This took years for AMD to blackbox the 386, and then:
"1987–1992: The arbitration proceeding, originally expected to take only six weeks, dragged on for nearly five years."
https://en.wikipedia.org/wiki/List_of_discontinued_x86_instr....
"The 80386 microcode was successfully extracted and publicly disassembled by a team of hardware historians and demoscene researchers (including reenigne, Ken Shirriff, and others). They extracted the 94,720-bit microcode ROM from 80386 die shots by combining image processing, neural networks, and human-aided automation. AI tools played a crucial role in cleaning up the die images, detecting cell patterns, and binarizing the data before humans parsed the 37-bit microinstruction formats. You can read about the full process on the Reenigne Blog.By contrast, the 8086 microcode was extracted through purely human-driven analysis of die photos. The 8086's 21-bit microcode is simpler and was fully reverse-engineered and disassembled in 2020. You can explore the decoded 8086 microinstructions interactively using the nand2mario 8086 Microcode Browser.8086 Microcode Browser - Small Things Retro - nand2marioDec 4, 2025 — Since releasing 486Tang, I've been working on recreating the 8086 with a design that stays as faithful as possible to the original...GitHub80386 Microcode Disassembled - Reenigne blogMay 23, 2026 — Well, they may have taken that as a bit of a challenge - they threw various bits of image processing, neural networks, and human-a...www.reenigne.org80386 microcode disassembled « Reenigne blog - daily.devMay 23, 2026 — 80386 microcode disassembled « Reenigne blog. A detailed account of disassembling the Intel 80386 microcode ROM, a 94720-bit blob ...daily.devi386 - WikipediaMicrocode reverse engineering In May 2026, the Intel 80386 microcode was reverse engineered and publicly disassembled by a group i...Wikipedia8086 microcode disassembled - Reenigne blogSep 3, 2020 — Recently I realised that, as part of his 8086 reverse-engineering series, Ken Shirriff had posted online a high resolution photogr...www.reenigne.orgThe 386 microcode has been fully reverse engineered - Reddit May 24, 2026 — In a group effort, a bunch of demoscene legends like reenigne have reverse engineered the microcode for the 80386, opening the pat...Reddit·r/thisweekinretro80386 microcode disassembled « Reenigne blog | daily.devMay 23, 2026 — 80386 microcode disassembled « Reenigne blog. A detailed account of disassembling the Intel 80386 microcode ROM, a 94720-bit blob ...daily.dev"