I spent a lot of time the last couple weeks working on getting the Dreamcast to output all kinds of high resolution video modes. It's part of DreamHAL, which I have written about here:
https://dcemulation.org/phpBB/viewtopic ... 9&t=105371
And is hosted here:
The files of interest are "startup_support.h" in the "inc" folder and "startup_support.c" in the "startup" folder.
Here's the list of modes I made. All non-HDTV 60Hz and 75Hz modes have been tested across 5 different kinds/brands/sizes of LCD monitors using direct VGA connections, and they all work. The ones marked as "for HDTVs" mean they are made according to HDTV timings, and are meant for something like a Beharbros Gekko box to connect to an HDTV through HDMI (and may not work properly through a VGA port). That said, all the HDTV modes have a non-HDTV equivalent.
And this is how I did it:
I've documented everything at length in the startup_support.h/.c files, so if you want to use them go for it!
A couple images I took of 1280x720 working are attached. It was pretty awesome seeing modes like 1280x720 and 1440x900 on my 4K monitor!