My understanding is that amnesia like you see in movies and on TV is so rare it's virtually non-existent. When someone's had an injury that causes extensive memory loss, it's a part of brain damage that causes other losses too, like being able to read, walk, or use the bathroom as needed. Recovery is not the ah-ha moment in entertainment, when memories flood back in, but a grueling relearning of what you used to know. Genuine memories tend to remain lost, but you replace them with what others remember of the same event, photographs, and suppositions.
Alzheimer's early presentation, in my observation anyway, is small lapses in memory. Not "Who am I, why am I here, what is this place?" but "What's the name of that company I worked for in the 80s, remind me how you're related to your Aunt Rose, and which way do I turn to get to church?"
As Alzheimer's advances, the losses are much bigger, whole swaths of memory gone, including who one's spouse or kids are, where this place is, what year it is, why you're there. It's heartbreaking.