If anyone wonders why it starts with a lot of character garbage on the screen: I can be wrong, but I think it is usually the decompression. The demo is only 4K in size, but gets expanded after loading so it fills the entirety of the available memory. Not to waste space for the decompression routine, it runs directly in the video RAM which is latter overwritten.
What you can see is literally program execution, with the fastest changing characters often being inner loop vars.
Yes; throwing the decompression routine and/or its variables into screen RAM also has the bonus of making it obvious to the user that something is happening without spending any time implementing a progress bar. Sometimes there's text like "Decrunching..." or "SuperPacker2.4 by c00lDude/TLA".
What you can see is literally program execution, with the fastest changing characters often being inner loop vars.