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Using my 21" Trinitron OSD, I can see that the resolution at boot is 720x400.

From memory, this weird resolution is extremely uncommon if ever to be seen as a choice on software that supports different resolutions, e.g. 640x400, 640x480 or 800x600 to name a few.

So why does this screen resolution exist yet is rarely used but at boot?

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This is historically not uncommon at all. Before starting an Operating System and initializing a graphics mode, your PC operates in the simplest display mode available, 80x25 text mode.

The original VGA 80 x 25 text mode (that is still supported by most modern graphics cards) has characters with a resolution of 9x16 pixels per character.

This adds up to 80 x 9 = 720 horizontal pixels and 25 x 16 = 400 vertical pixels.

Note: The character set commonly uses only 8x16 pixels per character. The 9th pixel column is a repeated 8th pixel column. This separates most characters for a nicer visual impression.

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    The 9th column in VGA text mode would show on areas made of block characters ░▒▓ (176/177/178). I made a special font where each line would have 3 dots, so they'd align on the 9-pixel grid. The 9th column would also be seen on Font based "graphical" cursor – Jonathan May 17 at 14:44
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    Only characters in the range 0xC0 to 0xDF get the 9th pixel column repeated, which not uncoincidentally is the box drawing character range. Text characters don't have this repeat so they can fill the entire 8x16 character cell while still having one column blank between characters. – Ross Ridge May 17 at 20:11
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    IIRC it also predates VGA .... you know there where CGA/EGA/MDA and herkules before VGA and the Text mode 80x25 was on EGA too and I think even CGA had that. – Spektre May 18 at 6:00
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    @Spektre, If you mean all those cards had a 80x25 text mode, you’re correct. But they all had different character / pixel resolutions. The MDA and Hercules had 720x350 (9x14 pixel characters), the CGA had 640x200 (8x8 pixel characters). EGA had 640x350 (8x14 pixel characters). – StarCat May 18 at 6:34
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    Yes, only VGA text mode has 720x400 active area resolution. And the text mode cannot be distinguished from other 400-line modes. – Justme May 18 at 7:35
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The PC boots in text mode, and on a VGA card the text mode indeed has a resolution of 720x400. As the text mode was the most used standard mode before GUI programs and operating systems were loaded, so it made sense to name the mode as 720x400.

However, the monitor actually cannot know what the exact resolution is, because for example 320x200 graphics mode has virtually identical sync polarity and timing.

So the monitor has to guess what it shows given the sync signal rates and sync signal polarity. Most likely it will show 720x400 with any 70Hz 400-line mode, such as 320x200 mode as well.

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  • 320x200 only has the same vertical timing as 720x400 if lines are double-scanned in 320x200 (which they are on VGA cards, but not on earlier cards), or if the 720x400 mode is interlaced (which I'm not aware of on any PC). – Michael Graf May 17 at 12:15
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    That is true, I just referred to the graphics modes that have 320x200 resolution. Of course the VGA monitor does not even support 200-line modes, so it is obvious from the context that this is about VGA so the 320x200 pixel modes are double-pixeled and double-scanned, so the monitor will regard them all as same 640x400 or 720x400 mode as they only differ in their timing by about 4 PPM. – Justme May 17 at 12:34
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Most of the typical full color resolutions were based on 4:3 ratio - e.g., 640x480, 800x600, etc. - which matched the aspect ratio of broadcast television at the time. Using a 4:3 pixel ratio on a 4:3 display screen meant you had have nominally square pixels. Graphics routines, especially with limited RAM & CPU cycles, are a whole lot easier with square pixels than non-square pixels. Those color graphics cards were typically memory limited in the old days. Memory was expensive, extra memory just for graphics on a card was even more of an extra expense.

However, the original IBM PC had a beautiful 720x350 monochrome 80x25 text mode using a 9x14 font. So it didn't need much memory and it pushed the limits of "ordinary" monitors, but on a real IBM monochrome monitor (or equivalent) it was fantastic. The pixels weren't square, but that really doesn't matter for a text-only display. (OK, the Hercules cards provided full bit-mapped graphics, but that was "business graphics", not playing games (mostly). When color graphics made its way from CGA to EGA to VGA, the 9x14 got bumped up to 9x16 and the total text mode resolution (for 80 cols x 25 rows) went to 720x400. This resolution fully utilized the capabilities of the VGA monitor at 70Hz, and finally brought the horizontal resolution of MDA text to color monitors, for the best of both worlds.

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  • I don't like the last sentence. It sounds like the text mode did not make full use of the 480 lines, which could be considered a bad thing. Actually, it is a good thing! The VGA monitor has a fixed horizontal frequency, so more lines means lower refresh rate. At 400 lines, it ran at 70 Hz, and at 480 lines, the refresh rate dropped to 60 Hz, which was flickery back in the time on many contemporary monitors. The 350-line EGA compatible mode ran at 70 Hz too (with longer blanking), because there was no need for higher refrehs rates at that time. – Michael Karcher May 18 at 19:09
  • In my experience, the problem with 60Hz refresh was often related to interaction with fluorescent lighting. In a dark room, a 60Hz monitor was fine :-) I didn't mean 400 out of 480 was bad, rather that it was good compared to 350 (of MDA) and it fit real well in terms of a x16 character cell. There just wasn't any need to go all the way to 480 since most text software would never use more rows at the time and graphics got the 480 anyway. – manassehkatz-Moving 2 Codidact May 18 at 19:34
  • I used to be annoyed as child by the 60Hz flicker during daylight in a 50Hz country, so it definitely was not an AC interference thing. But the amount of flicker depends very much on the phosphor type, and my first VGA monitor definitely was on the cheap end of the spectrum. May I suggest to change the last sentence to "This resolution fully utilized the capabilites of the VGA monitor at 70Hz, and finally brought the horizontal resolution of MDA text to color monitors"? It's not that 400 and 480 lines are just "close enough" - those are dedicated different modes of the monitor. – Michael Karcher May 18 at 19:44
  • OK. FYI, I am fully in favor of the collaborative nature of Stackexchange - i.e., unless something is contrary to my statements (which your comments are not) there is nothing wrong with editing it yourself. – manassehkatz-Moving 2 Codidact May 18 at 20:12

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