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Most VGA cards and monitors support additional undocumented resolutions, accessed by changing the VGA registers. Most are just variations on the regular modes (320x100 is the same as 320x200, but with 4x vertical scale instead of 2x), but some of them are quite odd, like 256x256, 256x224 and 400x300. So I wonder what the sync frequencies of these resolutions are.

This should be easy to find out using a monitor that has the ability to show these on it's OSD. If you need a program to enter such modes, Allegro 4's test.exe suppports a good bunch (select the "Mode X" driver).

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It's a bit more difficult.

The standard VGA registers allow a wide variation of modes and timings. Essentially you can use any frequencies you want, within the limits and resolution of your specific card.

Early analog monitors are different: Some have fairly tight restrictions on the frequencies and timings they accept. So you needed to program the registers correctly for your specific monitor.

As this wasn't very practical, there were three approaches to make it simpler: The first was to standardize monitor timings. The second one was for the monitor to tell the computer which frequencies it supported (the protocol is called Extended Display Identification Data, or EDID). The third one was multisync monitors, which accepted a range of frequencies, were fairly lenient about details in the timings, and offered monitor controls for fine tuning.

So the question "what are frequencies of common tweak modes" in this form doesn't have an answer. You can "tweak" the standard modes, keeping most of the timings. You can also figure out what timings your specific monitor supports, and make specific custom ones (I did this a lot, with even "funnier" resolution than the ones you mention; it's easy under X on Linux). Or, with a multisync monitor, you basically don't care.

But there are no specific frequencies that would somehow apply to all monitors.

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First and foremost, what you have listed are the visible active pixel and line counts. These alone cannot be used to determine the resulting H and V frequencies, as these depend on how many pixels per line there are in total between Hsync pulses and how many lines there are between Vsync pulses.

This information can be gotten by looking how the mode is set up, basically what is written as the H and V total counts are and what pixel clock they use (these bits are spread out to few registers). These resolutions sound like they are not using strict VGA sync rates and require a multisync monitor.

As first VGA monitors used fixed 31kHz Hsync frequency and three vertical scanning rates to allow 400, 350 and 480 visible line modes, many tweak modes adhere strictly to these compatible timings.

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