Extended memory

DOS programs, running in real mode or virtual x86 mode, cannot directly access this memory, but are able to do so through an application programming interface (API) called the Extended Memory Specification (XMS).

On a 286 or better PC equipped with more than 640 kB of RAM, the additional memory would generally be re-mapped above the 1 MB boundary, since the IBM PC architecture reserves addresses between 640 kB and 1 MB for system ROM and peripherals.

A supervising protected-mode operating system such as Microsoft Windows manages application programs' access to memory.

[1] A protected-mode operating system such as Microsoft Windows can also run real-mode programs and provide expanded memory to them.

[3] The difference is a direct result of the sizes of the values used to report the amounts of total and unallocated (free) extended memory in 1 KB (1024-byte) units: XMS 2.0 uses 16-bit unsigned integers, capable of representing a maximum of (65535 * 1 KB) = 64 MB, while XMS 3.0 adds new alternate functions that use 32-bit unsigned integers, capable of representing (4 G * 1 KB) = 4 TB (4 terabytes) but limited by the specification to 4 GB.

[2][3] (4 GB is the address range of the 80386 and the 80486, the only 32-bit Intel x86 CPUs that existed when XMS 3.0 was published in 1991.)

Extended memory is located above 1 MB, includes the high memory area , and ends at 16 MB on the Intel 286 and at 4 GB on the Intel 386DX and later.