Memory can be divided up into 5 areas: Conventional memory or DOS memory or Low DOS memory. This is the first 640K of RAM in your PC. DOS uses this memory to run programs, store data and store parts of the operating system.

Expanded memory is additional RAM that can be allocated for DOS programs. DOS was originally designed to use only 640K of RAM. This is known as the infamous 640K barrier. As software grew, so did memory needs. The designers of DOS added the expanded memory so DOS programs could have additional RAM - up to 32K.

Extended memory is any RAM above 1MB. This memory can be used for data and programs. Older DOS programs can not use extended memory, but must use conventional and expanded RAM.

Upper memory (UMA) or high DOS memory or reserved memory is between 640K and 1MB. This RAM is reserved for programs that control input and output devices.

High memory (HMA) is the first 64K starting at 1MB. DOS uses the RAM for memory resident programs, or device drivers.