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That mode is also non-square, with an aspect of 0.6:1. The IBM PC and its various video cards in the early days used a number of tricks to keep the cost down, one of those tricks is to keep the horizontal and vertical sync signals the same for different modes as much as possible. (Recently I’ve started to appreciate just how many tricks they used to keep the cost down, without sacrificing hardware reliability in the process!) For both 320x200 and 640x200, HSYNC ran at 15.75 kHz, VSYNC ran at 60 Hz. When you go from 320x200 to 640x200, all that happens is the pixel clock (the rate at which you read out from RAM) is doubled, so you get exactly two pixels horizontally packed in where there used to be one pixel. The older hardware, like EGA video cards, can only generate one other HSYNC speed: 21.8 kHz, for special 350-line modes. When VGA came out, it doubled the HSYNC frequency and, for these modes, would just read each row out twice. http://www.minuszerodegrees.net/video/bios_video_modes.htm So SCREEN 1 should have 1.2:1 ratio, and SCREEN 2 should have a 0.6:1 ratio. One of the better ways to verify this kind of thing is to compare box art from DOS video games to screenshots taken from DOSBox or sprites extracted from the data files. You can see that the screenshots from DOSBox match the sprite files, if you leave aspect unadjusted, but if you look at the box art from the retail packaging, you will (often) see artwork and screenshots with the correct aspect ratio, just like you see on real hardware. Here's a DOOM Wiki page discussing it, with a number of pictures and sources: https://doom.fandom.com/wiki/Aspect_ratio |