> After staring at instruction traces for a little while, I found the rather unexpected cause. In the DOSKRNL file (the DOS 4.0 kernel running inside OS/2 DOS b

> After staring at instruction traces for a little while, I found the rather unexpected cause. In the DOSKRNL file (the DOS 4.0 kernel running inside OS/2 DOS boxes), Microsoft tried to be clever and used an invalid instruction as a fast path to access protected mode services. But there was a fatal problem: Microsoft used the 0F 7F opcode, which is not undefined on Pentium MMX and later — it is the MOVQ instruction. For that reason, launching DOS boxes in build 6.78 fails miserably on Pentium MMX and later Intel processors (except perhaps on the Pentium Pro). Perhaps this is the origin of that anecdote about a meeting between Microsoft and Intel: https://devblogs.microsoft.com/oldnewthing/20041215-00/?p=37003 https://devblogs.microsoft.com/oldnewthing/20041215-00/?p=37...