Scroll engines

does nebody in this forum have a good scroll engine that can be used in platform games or sidescrollers? ALIAS is a good example of wut i want the scroll to be like or if u havnt played ALIAS it is like when the player reaches 1/4 of the stage it scrolls and if u go backwards it scrolls back.i’ve been tryin for a long time but hav been havin problems if u go all the back because the thing keeps on scrolling any help wud b great.