EMS was originally for the 286. It wasn't emulated by the CPU like you could on the 386. You bought an add-in board that contained the extra memory and handled

EMS was originally for the 286. It wasn't emulated by the CPU like you could on the 386. You bought an add-in board that contained the extra memory and handled memory requests on the bus for the bank switched area and accepted commands to change what areas the banked switched areas pointed to. When the 386 came out, it was common to have more than 1MB on the motherboard, but the motherboard (typically?) didn't natively emulate the EMS bank switching scheme, probably because at about that time, everyone figured that people were going to stop using DOS and switch to OS/2, and be running 32 bit code that could natively address higher than 1MB. When that didn't happen right away, Quarterdeck came out with QEMM/386, which used the Virtual 8086 features of the 386 to emulate EMS in software. Microsoft then shipped a not quite as good clone of QEMM as EMM386 with DOS 5.0.