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Extended memory (XMS) is RAM that is addressed above the first megabyte of the address space.

Expanded memory (EMS) is bank switched RAM that stays inside the 640K barrier.



> that stays inside the 640K barrier

Yes, but to be super pedantic, it stayed within the 1MB barrier[1] that real mode could "officially" address. It was perfectly okay to have your EMS window above the A000h segment in memory, i.e. above 640k, if no device was using it.

[1] Plus some change known as the "high memory area" (HMA), an artifact of how linear addresses are calculated from segment and offset.




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