How to modify a method of a Mojang class

[PlasmaPower]

I'm making a mod to multithread mipmap generation (speeding up startup and resource pack switching). This can be easily achieved by modifying the loadTextureAtlas method in TextureMap. My question is how do I do this? The TextureMaps are instantiated by calling new TextureMap(args) in Mojang's code. The texture map for blocks is instantiated in startGame(). I've looked into ASM some, but I can't figure out how to use it in combination with MCP and Forge. The ASM page on the Forge wiki is blank.

[PlasmaPower]

Thanks! I'd looked into creating a coremod, but as I understand it that only loads a mod before Minecraft is loaded and I could not find documentation on replacing classes. The video looks promising, I'll look into it in a bit.

[PlasmaPower]

I think I'll be using ASMHelper: https://github.com/squeek502/ASMHelper

[LexManos]

You dont.