How to find edit enemy health with FCEUXD:
by bbitmaster

Enemy health is extremely easy to find and edit in almost any game with FCEUXD.
In this tutorial, I'll take a completely random game, and show you how it's done

For no particular reason, the game I chose was Ninja Gaiden 1. So lets get started.

We'll start by trying to find and edit the first boss's health in the rom. Before
we can do that though, we need to know where his health is at in ram. So, Start a
new game, and fight your way through the first level, until you get to the door
that leads to the boss, be sure to make a saved state before going through this door

Using the cheat console isn't really that hard once you get the hang of it. What
you're trying to do with is is search for values in the ram. We assume his health
is a value that decreases each time we hit him. Although I have seen a couple of
games where the value actually increases, we will not assume this, because that is
rare.

Using the cheat search is something which is hard to guide someone through step by step,
because they may get different results each time. Here is what I personally just went
through to get the value. You can use this as a rough guide for finding enemy health in ram.

	1. Once I entered, the boss was in full health, so I opened up the cheat
	console, and hit reset.

	2. I hit him once, so that his health went down by 1 point, then, I pressed
	the less than button in the cheat console to bring it down to 154
	possibilities.

	3. Then, I hit him again so his health went down again, and pressed the less
	than button again. 63 possibilities now.

	4. I played around a bit in the game without hitting him, and press the equal
	to button (4 possibilities now)

	5. I see that out of the 4 possibilities listed, two of them are currently equal
	to the value $70, and $68. His health is much less than this, so I'll ignore them
	The other two are both equal to $0e, which is 14 in decimal. That looks to be about
	how many bars of health is on the screen for him. So it must be one of these.

	6. Upon adding the first one to the cheat list, it seemed to work, his lifebar
	didn't go down any, untill I hit him enough, then he suddenly died. so this must not
	be it.

	7. The second one worked great at giving him infinite health. So that must be it.

I strongly suggest you find this using the cheat search to get practice. But I'll tell
you the value I found anyway: $497

Now, most games set the enemy's health to specific value upon loading the enemy. After this
it has to detect when you hit him, and decrease it, untill it reaches 0. At this point
he dies. If we're wanting to change his health, then we must find out where it first loads
it at, so we can modify what it loads it with.

To do this, go back to your state that I told you to save before entering the boss room, and
open the debugger. Since you want to find out where the game stores his health, you must add
a write breakpoint to $497, and go through that door with it set.

The debugger should snap at this line:

$E28B:9D 90 04  STA $0490,X @ $0497 = #$10
That stores the A register into $497. Looking up a bit, we find 

$E288:B9 30 B5  LDA $B530,Y @ $B547 = #$10
This is what we're interested in, it loads A with $B547. So, that is what we want to change, $B547.

Open up the hex editor and scroll down to $B547 in memory. You can't change it from here, but only
from rom file editing mode. So right click on it, and select "go here in rom file." This should
take you to offset $3557 in the rom file. Try typing in 15, and go back to the debugger to turn
your breakpoint off (by double clicking on it), and hit run, and load your saved state.

Congratulations, the boss now has 21 healthpoints instead of 16.

If you roughly follow this method, you should be able to find and change enemy health in any other
game as well.

But we're not done yet! How would you like to give some regular enemies more health, so they take
more than 1 hit to kill?

First of all, what I have here is just a guess, I really don't have any idea for certain how the
game keeps track of this. I just know from what I have seen in other games, that they usually like
to keep variables like enemy health together, in a sort of array. When a new enemy is loaded, it
usually fills an entry in these arrays.

With this in mind, start a new game at the first level, then at the beginning of the level,
before any enemy has appeared, add a write breakpoint to $497. Before you even see the first enemy,
the debugger should snap at that same location
$E288:B9 30 B5  LDA $B530,Y @ $B533 = #$00
$E28B:9D 90 04  STA $0490,X @ $0497 = #$00

This time it's loading from $B533, if you go there in the rom file, you'll see that it's 00. If you
change it to 02, then those enemies will take 2 hits to kill.

You can play through any part of the game with a write breakpoint set on $497, and any time a new
enemy gets loaded into that position, you'll be able to easily find the offset for his health.

Alternatively, you can see from:
$E288:B9 30 B5  LDA $B530,Y
That the health, from all the enemies probably starts at $B530, and we know that this is at $3540
in the rom file. So You could change some random bytes there, untill you find the enemy you're
looking for

Anyway,

Have Fun!