Codex Gamicus
Conventional memory
Basic Information

Conventional memory on IBM PCs and compatibles represents the first contiguous 640 kilobytes (KB) of the main system RAM, as this is the maximum amount of system memory that can be directly addressed by the processor for user applications. While the Intel 8086 and later IA-16 processors could address 1 megabyte (MB) of system memory (which translates to 1024 kilobytes), the remaining 384 KB of addressable memory is called the Upper Memory Area (UMA), and it's reserved for system use and optional devices. In order for RAM above 1 MB to be accessible, the operating system (or shell) must be capable of running on an Intel 80286 processors or later using Protected Mode, although the first 64 KB beyond the 1 MB limit (called the Higher Memory Area, or HMU) could be accessed under Real Mode.

Conventional memory, due to its very limited size, is a very important consideration when playing video games under MS-DOS, since MS-DOS must leave enough conventional memory for the video game to run correctly.