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!

Submitted by: John Carpenter

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

GOD MODE:

if you want to look at a level of doom before playing it and dieing 
use god mode code: 

down,L2,square,R1,left,L1,right,circle.

Make sure when you start the level push start then use the code

Submitted By: L Hendrix


IT ENABLES YOU TO ZOOM IN AND OUT OF YOUR MAP:

Press the 'select' button on the controller to receive the map, then to
zoom in, press and hold L1 and x together.  To zoom out, press and hold R1
and x together.  Now you can find it easier to see where you are going on
the map.  Cool or what!

Submitted and FULLY tested By: Jack Williamson