Extended memory, also known as XMS (eXtended Memory Specification) is a technology that enables capacity above the 640KB standard MS-DOS limit of main memory.
Extended memory was introduced in the Intel 80286 processor, increasing the amount of addressable memory beyond what was introduced in the similarly named expanded memory (EMS). Prior to XMS, expanded memory was used to increase the 640KB upper limit of DOS to 1MB using a gated memory card riser.
Extended memory initially boosted the addressable memory to 16MB. XMS 2.0 extended the capacity to 64GB and XMS 3.0 to up to 4GB. Beyond XMS 3.0, AMD64, the de facto x86-64 standard, expands maximum memory to 256TB.
Programs written to use expanded memory would be redirected by driver emulation to use extended memory’s upper memory area instead. This process involved no special hardware, just increased RAM and support in the processor design.
Extended memory can only be addressed by software running in protected mode as all but the small portion of upper memory are unreadable by programs running in standard real mode.
Fun fact: The 640KB upper limit of DOS is the alleged source of the often-quoted but steadfastly denied Bill Gates statement: “640KB ought to be enough for anyone.”