Doom cheat codes

Original game
To use a cheat code in most versions of the game, one simply types the code while playing (this also applies while in automap mode). After the last letter has been entered, a message of acknowledgement will appear at the top of the screen. The status bar, face, and ingame view may also change, depending on the effects of the code. All of the codes for the original game work in all ports, unless otherwise noted.

Cheat codes in vanilla Doom are disabled on the Nightmare! skill level, except for the iddt and idclev cheats. However, many source ports enable cheats for all skill levels.

In some ports and in the original executables, cheat codes may be different on non-QWERTY keyboards. For example, in France, idkfa may be idkfq, because of the AZERTY keyboard layout commonly used there.

All Doom versions

 * 1) Prints the message "Doesn't suck - GM". Taylor's explanation for this message: "Choppers is a State Programming Contest game that a friend of mine named Guy Maor (GM) helped write. He's a little defensive of it 'cause it wasn't *quite* finished on the release day. Was a cool two-player Rescue Raiders rip-off. He was drunk and talked to me one night, and I thought I'd put it in."
 * 2) CLEV stands for 'change level'.
 * 3) Taylor and two fellow hackers made up an informal fraternity called Delta-Q-Delta (hence the DQD) during their college days. The "Q" stands for "quit", and represents the grade members should strive to receive in their classes.
 * 4) According to the function P_DamageMobj in p_inter.c, "invulnerability" only protects the player from attacks doing less than 1000 points of damage, whereas a telefrag inflicts 10000 points. This also means that DeHackEd can be used to circumvent the God mode cheat by creating incredibly powerful weapons.
 * 5) DT stands for 'Dave Taylor'. The code can be used in cooperative multiplayer games by first pressing T and then typing the code while holding ALT.
 * 6) FA stands for 'full ammo'.
 * 7) KFA stands for 'kick fucking ass', Killer Fucking Arsenal or 'keys & full ammo'.
 * 8) MUS stands for 'music'. This cheat only works in Doom v1.666 or later, in Ultimate Doom, IDMUS40 - 45 will play music from intermission, main screen and end screen, in Doom 2, this is 33 - 35.
 * 9) MYPOS stands for 'my position'.
 * 10) SPISPOPD stands for 'S'mashing 'P'umpkins 'I'nto 'S'mall 'P'iles 'O'f 'P'utrid 'D'ebris. See more in SPISPOPD article.

Doom 95
The prefix "fh" stands for the initials of Fred Hommel, the Doom 95 author who created the codes.

The "hall" in "fhhall" refers to Jason Hall, CEO of Monolith, who requested that particular cheat (hence the "BY REQUEST..." message that displays when the code is executed). The actual damage done to each monster is 10,000 points.

Trivia
If you type fhshh, you get the message, "Be vewy vewy quiet...". This is a reference to Elmer Fudd's catchphrase, "Be vewy vewy quiet. I'm hunting wabbits." in the beginning of a Looney Tunes or Merrie Melodies animated short.