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I learnt that there are 4 video modes as described on wiki.
I came across many games for the PC-88, like Thexder (1985), which are in the aspect ratio of 4:3 and in more than 2 colors.
I don't understand how this works and can't find a resource describing it.
My guesses are, games work like this:
Thank you for your help!
I don’t know about all PC-88 games, but Thexder at least runs in 640×200 V2 mode with 8 colours, with dithering (which can also be seen in the PC version, which runs in 640×200 EGA).
Screen resolution and aspect ratio are independent: you can assume in most cases that images will be displayed in 4:3 on CRTs of the time, which implies that pixels are stretched if necessary. 640×200 results in vertically-stretched pixels, and will be familiar to users of CGA computers (of which I imagine there are more here than PC-88 users). You can see this in properly-scaled screenshots of Thexder, for example its title screen:
640×200 is basically 640×400 with each row of pixels repeated vertically by the electron beam. Since you're repeating pixels without using more memory, you can devote more memory to each pixel to provide more colors. 2 colors needs 1 bit of memory for each pixel (21=2), while 8 colors needs 3 bits per pixel (23=8).
