story identification - Looking for a YA sci-fi book with a girl magically rescuing a boy trapped in a computer. 80s, I think

I can't recall either title or author, and think I read it in the 80s or VERY early 90s. The plot had a young girl discovering magical ability, and contacting a boy with similar talents. He ends up captured and imprisoned in a computer core, being used to power it. The girl has to touch points on a circle in a pentacle pattern before he completes the circle, or he'll be trapped and the bad guys take over the world or something. I remember the book used BASIC language commands, like GOTO10, for a programming loop. I seem to recall the book had a white cover with a very simple and small printed title with a small pentacle.