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.
"Out of the Dark World" by Grace Chetwin
When a boy whose mind is trapped inside a computer program begins appearing in her dreams, sixth grader Meg uses a hypnotic trance to summon the sorceress Morgan le Fay for aid in freeing him.