How to initialize video memory?

[γRB]

I've seen quite a lot of codes replacing certain on-screen values in game, which is the easier than actually "drawing" a space and dedicating an address or replacing another one? What I'm asking help with is.., for instance to print a certain number or value into actual "text on the screen". I've seen not so many codes like that and I actually kept this doubt for myself for quite a lot of time. So I know it is possible, but how? Using MEM90?