How to Hack NES ROMs

This was written by: Tony Hedstrom

Make sure that you read the "Updates" at the end of 
this document.

This document explains in detail how to hack a NES 
ROM.  When I say "hack", I mean that you will be
able to change a NES ROM (or game) to permanently
include the effects of any Game Genie code!  Which
means that every time that you play the game that you
hacked, you would get the effects of the GG codes,
but you wouldn't have to enter any GG codes!  

As far as I know, this method will work on either
6 or 8 digit codes.  If you are using an 8 letter code,
make sure that you read the updates at the end of this

The main reason that I'm writing this is mostly for
educational purposes.  You can achieve almost the
same thing by just entering the actual Game Genie
codes, but you won't learn anything that way.  If
you use the method that I describe here, you will
learn a great deal about how the real Game Genie
actually works.  

It is actually very self satisfying, and pretty cool
to be able to open up a game ROM, which has thousands
and thousands of letters and numbers in it, and by
changing just a couple of those letters or numbers,
you can get the same effect as any Game Genie code!

For this example, I'm going to use the Super Mario
Bros ROM (which has no title in it).  The first
version of SMB that I tried (which had a title in it)
didn't work right.  I'm not exactly sure why the
first version that I tried didn't work, but it may
be because the person who made the ROM didn't do it
correctly.  So if you are having trouble getting this
method to work on your ROM, try getting a different
copy or version of that ROM.

Here is a list of what you will need:

1) A NES emulator (I use NESticle).

2) The ROM (or game) you want to hack.

3) A Hex editor (I use Hex Workshop v 2.54).  You can
   get a free trial version of Hex Workshop at:

4) A program which will convert GG codes into a .NES
   Hex address (NOTE: The .NES Hex address is not the
   same as the ROM address!  See explanation below).
   I recommend using the "Game Genie Decoder" program.
   You can download the program at:

   If that link is no good, try going here and click
   on the "Videogames" link:

   Or you can download it from my site, in the download

If you are using a NES GG code to Hex conversion
program, you will have to modify the Hex address
that the program gives you.  To modify it, just add
$10 to the ROM hex address.  The reason for this is
simple, most .NES games have a 16 byte header at the
very beginning of the game.

If you are using the "Game Genie Decoder" program
that I mentioned above, it will modify the ROM
address for you.  

Also, make sure that the Game Genie Decoder program 
is in the same folder as your NES emulator and ROM.
It should be named "GameGenie.NES".

One other note before we start: Some of the newer or
larger NES games use something called a "Mapper".
On some of these games that use Mappers, the ROM
address can be in more than one location.

OK, here we go: 

1) Start up your NES emulator (in this case, NESticle)

2) Load the Game Genie decoder ROM (program).  It
   should be named "GameGenie.NES".  You should see
   a screen similar to a real Game Genie code screen.

3) Enter the Game Genie code you want using the same
   method that you use with a real Game Genie.  Press
   the "Start" button when you are finished and the
   program will decode it for you.

   For this example, I'm going to use my "Start with
   900 seconds instead of 400 seconds" code for SMB.
   The code is: VGYOKK.  When I decode it, I get
   several numbers.  The only two that you need are
   the .NES Address (which is 1C8C), and the Value
   (which is CE).  Write the two numbers down.  If you
   are using an 8 letter code, you should also write
   down the "Compare Value" and then read the updates

4) Exit the emulator.

5) Open the ROM you want to hack (in this case, SMB)
   with your hex editor (in this case, Hex Workshop).
   Make sure your hex editor is setup to view offset
   addresses as hex (not decimal).

6) Find the .NES hex address that you wrote down.
   (It is best if you know a little bit about how to
   count in hex, but it's OK if you don't.)  Just
   scroll down the list watching the numbers on the
   left side of the screen.  When you see a number
   that is close to the number you wrote down (which
   in this case is 1C8C), stop scrolling.  (The
   numbers listed on the left side of the screen 
   always have the last number rounded off to zero,
   so the number I'm looking for is the number that
   I wrote down with the last number changed to zero,
   which would be 1C80).

7) Once you find the right number, use your mouse to
   click on any number in the same row (to the right).
   (These numbers are letters or numbers in groups of
   two, like 9D, 85, B1, etc...(these are just 
   examples, yours will be different)).

8) When you click on one of these two digit numbers,
   look at the very bottom of the screen where it
   says "Offset:".  Use the left and right arrow keys
   on your keyboard to move the cursor and watch the
   "Offset" number until it matches the number you
   wrote down before (in this case, 1C8C).  So when
   you have the right offset (in this example) you
   should see "Offset:00001C8C" at the bottom of the

9) Now, find the number you wrote down before for the
   "Value" (which in this case is CE), and using your
   keyboard, type in that two digit "Value".  (What
   you have just done is to replace the value in the
   game (ROM) with a different value. This is basically
   the same thing that a real game genie does!)

10) Exit the hex editor (it will ask you if you want
    to save the changes to the file, click "Yes".  
    It will also ask if you want to make a backup
    copy of the file, click "Yes" (in case something
    gets messed up, you will still have the original

11) Start up your NES emulator again.

12) Load the ROM (game) that you just hacked (in this
    case, SMB) and see if the GG code works (which
    means that the hack was a success!)  When I try
    the SMB ROM that I just hacked in this example,
    the time always starts at 900 seconds, instead
    of 400 but I don't have to enter any Game Genie
    codes!  Pretty cool.

If the above method didn't work, it could be because
the ROM you have may use a mapper, or it could be
because the ROM was not copied correctly, or it could
be because of some other reason that I'm not aware of.
Also, see the "Updates" below.

Also, if you are using a different hex editor than
"Hex Workshop", the above instructions may need to
be changed a little to suit your hex editor.

Anyways, I hope this document helps you understand
how the Game Genie works.

   Thanks, Tony Hedstrom

               May 18, 2000

UPDATE: I just learned some more information about how
the Game Genie works, so I'll include that info here.

If the above method didn't work, then try adding $8000
to the original address and try it again.  If that
doesn't work, then add $8000 again, and so on until
the hack works.

You will usually only have to do this on the larger 

UPDATE #2: This update has to do with 8 letter codes.
If you are using a 8 letter GG code, you can use the
"Compare" value (Check value) to help you find which
offset to change.  Do not change an offset unless it
matches the compare value (check value).  In other
words, check the offset from the GG code, and if the
value of that offset is not the same as the compare
value of the GG code, then add $8000 to the address
and see if it matches.  If not, add $8000 again and
see if it matches, etc...  Once the offset matches
the compare value, then that's the offset you want to

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