One tiny missing bit: drawing with XOR wasn't just to be able to easily recover the previous state. If you're on a black and white display and are dragging arou

One tiny missing bit: drawing with XOR wasn't just to be able to easily recover the previous state. If you're on a black and white display and are dragging around a rectangular outline, then dragging it over a filled portion of the screen makes it invisible. Even if it's only partly over a filled image, you can't see the whole thing. With XOR, the filled portions will invert, so you can always see the extent of the whole rectangle. Admittedly, if your screen is showing static (random noise), then you're still screwed.