Moon gravity
To use it, save it in your xkas folder (Where the xkas program actually is), use command prompt and type cd\program files\or whereever you put your xkas folder\your xkas folder. You want it to look like C:\Program Files\Applications\Xkas_v06>. Once you've got that > on the end of your xkas folder, type "xkas ASMfilenamehere.asm SuperMetroidfilehere.smc. It won't look like any thing happened, but it will apply the ASM code.

If you want to change what area the code applies changed gravity to, or to what extent the gravity is changed, open up the .asm file in notepad and there are instructions on the bottom that tell you how to do so.