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To add an existing GS/AR/PAR/CB cheat code, use "Gameshark Codes". Else, if you have a memory address and a given value for it, choose "Search Code" then "Add Code", then put in the address (as $XXXXXXXX), the byte range depending on the values your variable can take (1 is 0-255, 2 is 0-65535, etc), and the numeric value proper (in decimal, or hexadecimal if followed by a lower-case h). Various cheat types exist too (S(substitute on read), C(substitute on read with compare) or R(replace value before vblank) -- R being the default recommended option).
Searching for new cheat codes is also possible. The general idea being comparing variables, going back to gameplay so that this variable changes , then back to the cheat engine searching for any variables that changed in that particular way. For example:
* '''Infinite Health / Ammo / Time / Money:''' you start with 5 hearts, reset search (in the cheat engine, not reset the game you silly!), you get hurt to 4 hearts, search variables that decreased, get healed back to 5 hearts, search variables that increased, and so on. For the money, you can start with 0 when you have 0 gold and try earning/spending money. You could try using save states as well when you don't have as much freedom changing the value.
* '''Infinite Lives:''' It's game over when the internal variable for "Lives" is 0. So in some cases, when it shows "REST/LEFT=1" in-game and you lose a life then it becomes 0 but you're still allowed to play on your last life, that means the internal counter for lives has actually changed from 2 to 1 and not from 1 to 0, but in other games (where if you die on "LIVESx1" it's game over) it's not the case. In the case of lives, you actually know the exact value: reset search initially, then (if you have 2 lives on a game that allows you to play on "LEFT=0") search for variables with values equal to 3. Lose a life, search for variables equal 2, and so on.
The goal of these searches is to narrow down the list of results to a single or handful of addresses (variables) pertinent directly to whatever we're searching for. The search always starts with "Reset Search", going back in-game (Alt+C) to play and change stuff, then back to the cheat engine to compare the "Original" value with the "Current" value. Then you check the results page which must get smaller and smaller (if it's 0 results, you screwed up and need to reset search to try again). The comparing options offered by Mednafen are as follow, with O being the Original value, and C being the Current value (when you reset a search, they're the same):
# '''O==V1 && C==V2''' if you know the explicit numerical values for the variable , you write it here. The first time, just write the same value in both. Subsequent times, leave O blank and press Enter so that you don't write the old value again, and write the current value under C.
# '''O==V1 && |O-C|==V2''' same, but instead of writing the current value you write by how much it changed (0 if old value equals new value)
# '''|O-C|==V2''' you write by how much it changed (0 if old value equals new value)
