Link-up with 1 Disk
It is possible to play link-up Doom using only one copy of the game. It's a bit ropey but it does work and I think the main reason for that is that I am doing the swap method on one PSX (ie I have a Japanese and UK Playstation, a PAL copy of the game). Anyway, load up Doom on one of them, choose all the options and then start the linkup. It'll say "waiting for connection". Now wait a second or two, take out the CD and pop it in the other PSX and load it up. Set up the same options, and then start the linkup. It'll say "waiting for connection" then after a while it'll change to "loading" as will the other one. After another while it'll probably just stop. Wait until you can't hear the CD making any more noise than just the sound of inactive spinning and take it out and put it in the first PSX. Now wait a while and it should load up and work. With mine when a level is completed it'll then crash. I think it may be because of the linkup but I don't know. Anyway, it's good while it lasts!
More Codes
Note: To do the codes you have to pause the game then push these buttons
Invincibility: Down, L2, Square, R1, Right, L1, Left, Circle
All Weapons: X, Triangle, L1, Up, Down, R2, Left, Left
X-Ray Vision: L1, R2, L2, R1, Right, Triangle, X, Right
Complete Map of Stage: Triangle, Triangle, L2, R2, L2, R2, R1, Square
Items On Map: Triangle, Triangle, L2, R2, L2, R2, R1, Circle
Level Warp: Right, Left, R2, R1, Triangle, L1, Circle, X
Soundtrack
Play track two of the game disc in an audio CD player to hear music from the game.
Random levels
Enter DOOMMNLxxx or H3LL1SHxxx, where xxx the same character such as !!! or 666, at the password screen. The game will start at a random level with an unusual amount of ammunition, armor, and life.
Map all items
Pause game play and press Triangle(2), L2, R2, L2, R2, R1, Circle. To confirm correct code entry, a message will appear on the bottom of the viewing window and will disappear the game is resumed. All objects including enemies, items, weapons, and secret switches will appear on the map as blue triangles.
Map all lines
Pause game play and press Triangle(2), L2, R2, L2, R2, R1, Square. To confirm correct code entry, a message will appear on the bottom of the viewing window and will disappear the game is resumed. All walls and boundaries will appear on the map for the current stage.