Heads-up display

The heads-up display appears in the upper left corner of the screen, and (in vanilla Doom) consists of a one-line text message in red lettering.

A message is normally displayed in the following circumstances:
 * An item has been picked up.
 * A key is needed to open a door.
 * A cheat code has been activated.
 * In multiplayer mode, a player has sent a message, or left the game.
 * A game mode (messages, detail, gamma) has been changed.

Messages are displayed for approximately four seconds and then disappear. The "Enter" key will re-display the previous message. The "F8" key will disable messages (and subsequently reenable them).

In multiplayer mode, there is a second line in the heads-up display that is used to compose messages to be sent to the other players. The "T" key begins message composition, and a an underline (used as a cursor) appears. A message may then be typed, and sent with the "Enter" key, or one of several predefined messages (known as "chat macros" or "chat strings") may be sent by holding "Alt" and pressing the assigned number key. If there are more than two players, a message may be sent only to a specific player by using the first letter of that player's color instead of "T".

Many source ports have made enhancements to the heads-up display, such as scrolling message lines, additional game-generated messages, varying color and position of text, and a console mode.

Technical notes

 * The heads-up display code is also used for text messages in menu mode and in automap mode (including the level name).
 * Different messages are displayed when a key is needed for a local door versus a remote door: "You need a key to open this door" and "You need a key to activate this object".