(I think you're aware of this, but if not - please read)
It sets video screen memory outside of physical memory. This makes shifter display bus noise. By cleverly manipulating what shows on the bus, this noise is controlled and graphics are displayed. The advantage of doing it is you can modify what is being displayed just by reading memory (no write needed).
Description by demo coder:
https://beyondbrown.mooo.com/post/anatomy/
I hope Hatari can/will emulate this behaviour soon :)
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