I believe this is it: https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/faster-syscall-trap-redux/ https://blogs.msdn.microsoft.com/larryosterman/2005/

I believe this is it: https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/faster-syscall-trap-redux/ https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/fa... The IBM way required IBM BIOS and an absurdly long time (as I recalled it was hundreds of milliseconds, not just milliseconds) to re-enter real mode. The motivation was to be able to run DOS real-mode programs as well as protected mode programs.