The DOS version of Mortal Kombat II has a nifty hidden menu that allows various cheat/test settings to be activated. It looks like this:
To enable access to the hidden menu, a special key sequence needs to be input during the copyright screen (the white text on black background screen which is the first thing displayed when the game is launched). The sequence is AICULEDSSUL.
If the sequence is input correctly, a gong sounds, and then the hidden menu can be accessed at any point during the intro animation by pressing F9 (similar to pressing F10 for the normal options menu).
It’s a peculiar sequence to input during the few seconds that the screen typically stays on (you can read about its meaning here). Moreover, on some faster systems, the copyright screen disappears too quickly (after a fraction of a second), which is not enough time.
It is generally possible to pause the game at the copyright screen, input the code, then unpause it. This solves the timing problem, but you still have to memorize the code.
A different approach is to replace the long sequence with something short, and simple: a single key press. This can be done by a small hack in the MK2.EXE file.
MK2.EXE Hack Details
The game looks for keyboard scan codes, not ASCII values. The table of keyboard scan codes can be found in various places online, for example here.
Open MK2.EXE with any HEX editor, and search for the following string (which corresponds to AICULEDSSUL):
1E 17 2E 16 26 12 20 1F 1F 16 26
Let’s assume you want to set it up so that a single F9 keypress activates the cheat menu. Then replace the above with:
43 00 00 00 00 00 00 00 00 00 00
The first number – 43 – is the hexadecimal scan code for the F9 key, the zero after that indicates a sequence termination, and the rest of the zeros must be put there to preserve the size of the EXE file and not mess up any offsets inside it.
You can, of course, use any key or any key sequence of your desire, according to the scan code table. Also there are three zeros following the original sequence in the EXE file, so assuming one must be used as a terminator, it is possible to make the required sequence longer, up to 13 keypresses (instead of the original 11), although I cannot imagine why anyone would want to.
The sequence is the same in all DOS versions of MK2 – floppy and CD, so the hack is applicable to all of them.
Hidden Menu Functions
- Cheat Mode – master switch. Must be On for any of the others to have effect
- Freeplay – Unlimited kredits (continues) in 1-player mode (instead of the default 5)
- No damage to P1/P2 – Obvious
- 1 hit kills P1/P2 – Obvious
- Soak Test – Replaces the normal intro sequence with non-stop battles between kombatants. Normally one kombatant will fight all the others in turn. The name for this switch suggests that the goal of this was to find any possible bugs/instabilities in the game, involving any of the characters.
- Background – Forces the same background (kombat zone) to be used in all matches. The secret arena ‘Goro’s Cave‘ can also be selected. Note that this has no effect on the secret battles and on the boss battles. These will be fought in Goro’s Cave and Kahn’s Arena, respectively, regardless of the selection here.
- Battle Plan – Allows starting the 1-player ladder anywhere. Options are: 1-11 for the respective levels, Mystery (Question Mark – always fighting Smoke), Shang Tsung, Kintaro and Shao Kahn. It would be nicer if selecting Mystery would allow some randomization of the secret opponent, but at least it is always possible to get to Jade by selecting 11 and winning one round using Low Kick only (the normal condition).
- Drone Ending – Force the AI opponents to always perform a chosen finish, instead of randomly / not at all. Options are: Fatality 1, Fatality 2, Friendship, Babality.
- Stop Clock – When On, the timer is disabled, and the match is always fought until one opponent is out of health.
- Set Drones – When selected, replaces all AI opponents with the given character. Probably was chosen to allow another variant of soak testing. Unfortunately, only the 12 normal fighter can be selected here – not the bosses, nor the secret characters.
All in all, the hidden menu of the DOS version is very nice and provides more flexibility / cheat options than the Arcade and Console versions.
Cloaked Thargoid 
That’s really helpful, thanks!