tldr: https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93... Intel screwed up implementing v86 mode by missing one privileged accesses trap - Protected Mode program (CPL 3) executing CLI and expecting POPF to restore interrupt flag. CLI will correctly trap, but POPF is silently ignored AND doesnt trap. No easy workarounds available.
That's in protected mode. In V86 with IOPL < 3, both PUSHF and POPF will trap.
However I think some versions of Windows used to run V86 code with IOPL = 3. At least I remember being able to run "CLI, HLT" instructions in DEBUG inside a DOS box and locking up the entire system.
"Intel screwed up implementing Protected Mode by missing one privileged accesses trap - Protected Mode program (CPL 3, IOPL < 3) executing CLI and expecting POPF to restore interrupt flag. CLI will correctly trap, but POPF is silently ignored AND doesnt trap. No easy workarounds available."
One of the prominent victims is Doom. Works in Windows only due to hacks.
https://news.ycombinator.com/item?id=35919612
tldr: https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93... Intel screwed up implementing v86 mode by missing one privileged accesses trap - Protected Mode program (CPL 3) executing CLI and expecting POPF to restore interrupt flag. CLI will correctly trap, but POPF is silently ignored AND doesnt trap. No easy workarounds available.