In the PC Doom/Doom II universe the term "demo" refers to a file that contains
a recorded session of gameplay.  Demo file names have a ".LMP" file extension,
and so are sometimes referred to as "LMP files" or "LMPs".

Despite appearances to the contrary, a demo file is not a video or "movie" of a
game; It is simply a basic data file containing encoded keyboard/mouse/joystick
input, which was recorded in precisely-measured intervals during a live game.
This block of recorded input is stored between a small game-information header
and an end-of-file marker.

When a demo is "played back", an actual live game begins, and control of the
player character(s) in the live game is provided by the keyboard/mouse/joystick
input recorded in the LMP file.

In the Doom/Doom II game engine, the simulation of randomness is influenced by
signals from input devices.  When a demo is played back, the recorded input is
read from the file, and gameplay is affected accordingly.  This is why monster
activities and other "random" events will always occur the same way every time
a given demo is played back.

To record or replay demo files, a commercial version of the compatible Doom/
Doom II software is required.  (A demo file is only an optional accessory to a
Doom/Doom II game, and is not executable by itself.)

For best results when recording any demo, avoid pausing the game, opening menu
screens, or entering cheat codes while playing.  (Cheat codes and some other
types of input are not stored while recording demo files, and this missing data
will often cause demos to go out of sync during playback.)

For best results when replaying a demo, be sure to use the exact same version
of Doom/Doom II that was originally used to record the demo; otherwise the demo
may fail to load, or may go out of sync during replay.  For example, a Doom II
demo cannot be replayed using Doom 1; a demo recorded using Doom II v1.9 cannot
be replayed using Doom II v1.666.

For information on playing demos when using DOS or Doom 95, click here.  For
more about demo command-line options and parameters, click here.

If you are using a source port (a third-party Doom/Doom II engine), note that
demos made with the original engines may go out of sync when they are replayed;
and some source ports will not load/replay/record original-format demos at all.
PrBoom is one of the more reliable modern source ports for playing Doom/Doom II
demos made with the original engines; Chocolate Doom is another.

For general notes on the demos available at this site, click here.
Back to Doom Help page / Classic DOOM home page