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Intel 286 secrets: ICE mode and F1 0F 04

Accepted submission by owl at 2024-02-07 19:59:20
Hardware []

With some recent interest in undocumented opcodes and microcode on modern Intel chips, I decided to do a proper writeup on what I found out while researching and playing around with the venerable 80286 ("Beige Unlock"?).

ICE mode

The documentation for the '286s LOADALL instruction - which Intel only made available under NDA back in the day - briefly mentions how it is used during automated testing of every produced chip (and is thus guaranteed to work). But its other purpose was kept secret: to support In-Circuit Emulation (ICE).

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