186 didn't have protected mode :) On the 286, the only way out of protected mode was via a CPU reset. The keyboard controller (which was a separate chip on the

186 didn't have protected mode :) On the 286, the only way out of protected mode was via a CPU reset. The keyboard controller (which was a separate chip on the motherboard, an 8042 microcontroller) had a documented command to do that, and one of the first things the BIOS entry point did was to check a flag indicating that the reset was intended to exit from protected mode, if so it gave control back to the application by jumping to a pointer it provided. Another way to reset the CPU was to cause a "triple fault" - an exception that can't be handled will try to invoke the double fault handler, when that also fails the CPU shuts down, sending a special bus cycle that external hardware would detect and trigger the reset. This actually became the preferred method since the keyboard controller was quite slow. So slow that it gives you enough time to run an undocumented instruction which dumps the entire CPU state: https://rep-lodsb.mataroa.blog/blog/intel-286-secrets-ice-mode-and-f1-0f-04/ https://rep-lodsb.mataroa.blog/blog/intel-286-secrets-ice-mo...