Note: Not mine, author made this and is very educational, even image is static.

PS: I was so happy to watch these two videos which is two parts, very good details, most importantly: Fully filled out, I repeat; Not black borders at all! Despite this, the ROM has access time that delayed caused black vertical lines every few pixels horizonally. A good lesson on related to conversion design from any resolutions to the fixed resolution display like LCD rather than analog CRT.
I hope this can help or inspire others to make better converter so that works properly to output fixed resolution with fixed Hz (especially LCD monitors) from any resolutions with different Hz.

https://www.youtube.com/watch?v=l7rce6IQDWs

https://www.youtube.com/watch?v=uqY3FMuMuRo

Cheers,