This is for Doom version 1.2 shareware. These codes are in HEX offset.
This may come in handy since you can't use the cheat keys in Nightmare mode.
Start a new game, save it and then exit.
Open the savegame file (doomsav0.dsg if this is your first game).
Use a hex editor to change the following:
Everything you will need to change is in the first 0 to 200 offset.

WARNING! If you are hacking up a savegame file you don't want to lose, 
         Make a backup of it first. 


   Offset 0:       Range          Function    


   52 - 53     ?                  Health
                                  I can't get this to work the way I
                                  want. You can mess around with it if
                                  you want to. When I set it over 100
                                  it didn't do any good.
                                  

   56 - 57     00 00 - FF FF      This is your armor, different
                                  from health, you can make this
                                  as high as you want!

Example: 9999 in hex is 27 0F 
Remember to always flip the two numbers around.
Go to 56 and change it to 0F and then go to 57 and change it to 27
Now you have 9999 armor.


For the guns and cards, 00 = off and 01 = on


   76             00 - 01         Blue Keycard,                    
   7A             00 - 01         Yellow Keycard
   7E             00 - 01         Red Keycard

Example: If you want the red card, go to 7E and change the 00 to 01.


The Pistol is always on.

   B2             00 - 01         Shotgun,                               
   B6             00 - 01         Chaingun
   BA             00 - 01         Rocket Launcher                                                 
   C6             00 - 01         Chainsaw

The Plasma Rifle and BFG9000 are not in the shareware version. 
Even if you turn them on, there are no sprits for them.

Example: If you want the Rocket Launcher, go to BA and change the 00 to 01

   CA - CB     00 00 - FF FF      Bullets
   CE - CF     00 00 - FF FF      Shotgun Shells
   D6 - D7     00 00 - FF FF      Rockets
   DA - DB     00 00 - FF FF      Maximum Bullets
   DE - DF     00 00 - FF FF      Maximum Shells
   E6 - E7     00 00 - FF FF      Maximum Rockets

Example: 9999 in hex is 27 0F 
Remember to always flip the two numbers around.
For 9999 max bullets, go to DA and change the number there to 0F and 
then go to DB and change the number there to 27 Now you have 9999 max bulets.


   For God mode, go to F2 and change the  00 to 02

   For Light visor, go to 72 and change the 00 to 3c
   then go to 73 and change the 00 to 10

   For the radiation suit, go to 6A and change the 00 to 0F
   then go to 6B and change the 00 to 07


That's it for now. Have Fun!