> that put processors capable of protected mode (the 286--sort of--and later) into protected mode temporarily.
Or, on the 386 and later, permanently, running DOS as a vm86 task instead. EMM386 and similar did that. It was very transparent, so you usually wouldn't notice, and even for programs that would like to drive protected mode themselves, there was an interface called DPMI (earlier VCPI) with which they could wrestle protected mode away from the memory manager (being the exception to "permanent").
DOS4/GW, which most probably know from DOOM and other games, used that interface.
For software that did not use DPMI/VCPI, you usually got a message like "CPU already in protected mode" and had to disable your memory manager (usually by rebooting).
Or, on the 386 and later, permanently, running DOS as a vm86 task instead. EMM386 and similar did that. It was very transparent, so you usually wouldn't notice, and even for programs that would like to drive protected mode themselves, there was an interface called DPMI (earlier VCPI) with which they could wrestle protected mode away from the memory manager (being the exception to "permanent").
DOS4/GW, which most probably know from DOOM and other games, used that interface.
For software that did not use DPMI/VCPI, you usually got a message like "CPU already in protected mode" and had to disable your memory manager (usually by rebooting).