The [https://www.cpcwiki.eu/forum/programming/interrupt-positions-with-various-sized-screens/ IntPos tool] by Kevin Thacker can help visualize the potential split screen areas.
Note: if all you want is multiple graphics modes in the same frame, you don't need to touch the CRTC at all. More information here: [https://www.cpc-power.com/cpcarchives/index.php?page=articles&num=184 Multi-Mode Graphique (FR)]
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