Games intended for remote play through a dial-up bulletin board system -- sometimes multiplayer (though rarely simultaneously) these were primarily textmode and fed only the slimmest streams of data back and forth due to the narrow bandwidth modems provided. A door is a computer program, on a bulletin board system, that runs outside of the main bulletin board program. Sometimes called external programs, doors are the most common way to add games, utilities, and other extensions to BBSes. From the 1990s on, most BBS software had the capability to "drop to" doors. Several standards were developed for passing connection and user information to doors; this was usually done with dropfiles. Most doors were responsible for operating the serial port or other communications device directly until returning control to the BBS. Later development of FOSSIL drivers have allowed both BBSes and their doors to communicate without being responsible for direct operation of the communications hardware.
