The video card doesn't output 240p. It line-doubles it to 480p. So that's the first part of the question answered.
Most LCDs still have an option hidden away to either stretch or pillar-box resolutions that aren't 16:9. Turn that on and you'll get the natural 4:3 picture.

Ah great, that's good to know! So VGA graphics cards have a minimum resolution of 640x480 at all times?
320x240 without line doubling was EGA or CGA then and VGA added line doubling to provide backwards compatibility?

VGA 13h (320x200) is recognized as 720x400 text mode typically. 240p is not a VGA really.

CGA - 320 x 200 or 640 x 200 (there was also a funky 160 x 200 or 100 but nobody cares, and it was largely unsupported).

MDA - character graphics only. Hercules added graphics, 720? x 348.

EGA - supported well enough standard cga modes. There was also the enhanced 640 x 350 16 (or 8?) color mode. Aftermarket EGA clones bumped the res up to 800 x 600 and perhaps more.

PGC - came with a built in CGA emulator card (in loose terms). Added 640 x 480 256 color graphics. Could be programmed like a standard video card I suppose, but generally it had a weird dialect where you spoke to the card in terms of geometric constructs. There were 2 clones, 1 at least is missing the discrete cga board, but I have to believe the ability to emulate cga was still there. The Vermont Microsystems card I own. It's a sandwich of only 2 boards. I know nothing about the Orchid? pGC clone.

I'm done.

When playing DOS games on a VGA adapter, you may meet games that output effectively 350 lines, 400 lines, 480 lines or more.

For example: here is Sim City. It runs in a standard video mode "10h" that was first introduced by the EGA video adapter. It is 640x350 @ 16 colors:

Standard video modes that had fewer than 350 horizontal lines are "line doubled" by VGA adapter, to give better visual quality. For example, the 320x200 video mode is actually clocked out as a 320x400 video. Here is Keen 5:

Here is the famous Mode-X 320x240, which is clocked out as a 320x480 video mode:

So the program had a framebuffer of 320x240, but video adapter scandoubles the output to 320x480.

This scandoubling is not mandatory though. The register space of a VGA adapter can be manually programmed to output 320x200 without this scandoubling active, and the effect is that it then doubles the refresh rate. I can't recall of games or applications that would have done so, but for example the Luminati demoscene demo does this kind of an effect, resulting in a 320x200 120 Hz refresh rate (it does a couple of other mode tweaks as well):

Also, an application may skip the 320x240 -> 320x480 scandoubling, and *actually* utilize a 320x480 framebuffer to keep refresh rate intact. Quake for example supports this:

Anyhow, all of this scandoubling details is possibly a distraction. The rule of thumb with standard video modes is that video modes < 350px tall were practically always scandoubled, whereas 350 lines modes and higher were not.

Also of note, the regular DOS 80-column OS text prompt was 720x400 pixels in size, not scandoubled.

I've got an ASUS PA248QV display that I mostly test vintage PC stuff on, and this display requires at minimum a 640x400 video mode, or refuses to sync.

oldhighgerman wrote on 2024-08-28, 19:53:

CGA - 320 x 200 or 640 x 200 (there was also a funky 160 x 200 or 100 but nobody cares, and it was largely unsupported).

The funky 160x100 mode is double-scanned, so it is a 200 line mode, just like all other CGA modes. It was not supported by the IBM BIOS and IBM did not provide any instructions on how to enable that mode with their documentation. It was used in some obscure games.

oldhighgerman wrote on 2024-08-28, 19:53:

MDA - character graphics only. Hercules added graphics, 720? x 348.

Correct.

oldhighgerman wrote on 2024-08-28, 19:53:

EGA - supported well enough standard cga modes. There was also the enhanced 640 x 350 16 (or 8?) color mode. Aftermarket EGA clones bumped the res up to 800 x 600 and perhaps more.

EGA supported all official BIOS-supported CGA modes, as well as 16-color modes of 320x200 and 640x200 on all monitors. On special EGA monitors, you got a better looking text mode and a 640x350 graphics mode. While the EGA card allowed "choose 16 out of 64 colors", the IBM ECD only supported 64 colors in the improved text mode and the high-res graphics mode. The original IBM EGA had a base configuration of just 64K video RAM. In that configuration, the 640x350x16 mode got downgraded to 640x350x4.

oldhighgerman wrote on 2024-08-28, 19:53:

PGC - came with a built in CGA emulator card (in loose terms). Added 640 x 480 256 color graphics. Could be programmed like a standard video card I suppose, but generally it had a weird dialect where you spoke to the card in terms of geometric constructs. There were 2 clones, 1 at least is missing the discrete cga board, but I have to believe the ability to emulate cga was still there. The Vermont Microsystems card I own. It's a sandwich of only 2 boards. I know nothing about the Orchid? pGC clone.

There are some more clones, for example by Matrox.

I have a Multibus Matrox video card. When I bought a whole boat load of Multibus cards, I found it and nearly passed out. It don't count of course. Mb is so obscure, not pc compatible (despite using 8088/6s, 80188/6s, etc.), amd pretty rare that no one gives a damn frankly.

There were other isa video cards - Plantronics, Persyat BOB, Hercules InColor, Sigma 400, Number9 Revolution, etc. Some supported PC modes. Some were funky as all get out. The last one didn't come with ANY drivers. I always wanted one. Then realized one day I had one.

A monitor has to have the ability to sync to 120hz. Today it isn't a big deal, but in the 80s I'd have to say nearly none did.

720 x 400 was a vga mode. Not present in early MDA, but not sure about Herc cards. I love the 5151 monitor, prefer it easily above a CGA. But am MDA card is a poc. Hercules is a better card. And IINM supports cga games on a 5151???

To add a little bit more to the confusion, mode 13h (320x200 256c) was originally being known as "PS/2 graphics" or "MCGA".
That's what many game boxes had written on them when they were referring to 320x200 256c graphics.
You can still see them at the Mobygames game database, for example.

"VGA" was rather known for its 640x480 pels resolution, often with 16 colours or monochrome (mode 12h, mode 11h).
That's what non-game software meant by "VGA". 720x400 pels text-mode also was sometimes called "VGA text-mode".

However, nowadays very little people make a distinction between MCGA and VGA anymore.
Probably also because VGA is being required by games which tweak mode 13h, which blurred the lines.
Once the mode is tweaked, it will no longer work on the real MCGA hardware.

Anyhow, from a technical point of view, mode 13h is atypical to VGA.
VGA is like EGA, a highly complex circuitry with lots of planar video modes and bank-switching capabilities.
This is reflected in mode 12h, for example. 640x480 in 16 colours, as we know it from Windows 3.x.

MCGA's mode 13h, by contrast, is a very dumb, yet very effective frame buffer that can hold everything in just 64KB of video memory.
64KB.. That's the size of one x86 segment. mode 13h, when unaltered, does not require any bank-switching.
It's directly addressable, with each byte (8-Bit) holding 256 values, one for each of the 256 standard colours.

(Alternatively, MCGA hardware can also do basic CGA modes
and 640x480 in monochrome, mode 11h, another plane-less mode that fits in 64kB.)

So in this respect, mode 13h is a bit like an alien/an anomaly to the EGA/VGA architecture.
That's why I think that "MCGA" as a term for mode 13h is still being deserved.

Unfortunately, many variations of mode 13h do exist, making it difficult for users to differentiate.
Mode X, Mode Y, true 320x400 mode with actual image data in the normally doubled lines, default mode 13h but with a custom palette, palette-cycling etc pp.
And they're nolonger MCGA compatible. Not that this matters much, since merely two different computers had MCGA hardware, but..

Edit: This is just an oversimplification, it's not the whole story. The subject of VGA programming does fill entire books.
I've just meant to highlight the relationship between MCGA and VGA a little bit. 🤷

oldhighgerman wrote on 2024-08-28, 23:08:

720 x 400 was a vga mode. Not present in early MDA, but not sure about Herc cards. I love the 5151 monitor, prefer it easily above a CGA. But am MDA card is a poc. Hercules is a better card. And IINM supports cga games on a 5151???

Actually, it's rather simple. There are mode numbers for both graphics modes and text-modes and it's up to the BIOS to set it up as needed.
A VGA card has VGA BIOS, which replaces parts of PC BIOS code.

By default, VGA cards behave pretty much like MDA to software (80x25 default resolution). It's mainly that they have a different font and font resolution.
-> Edit: Early MDA cards were in colour, actually. They had RGB internally, but it wasn't officially being used.

MDA/Hercules use the default font (9x14 pixel) with codepage 437. Or if the card has a ROM chip installed, it may use an slightly alternate font, say for Greek users.
VGA cards read the VGA font (9x16 pixel) stored in the VGA BIOS ROM chips, but the fonts can also be altered/replaced in memory later on.
The graphical WinBIOS user interface makes use of this, for example. Or Central Point's PC Tools 7. They upload custom fonts to draw their TUI (text-mode GUI).

EGA and Hercules Plus also had a soft font feature, I believe. Hercules InColor, too.

oldhighgerman wrote on 2024-08-28, 23:08:

I love the 5151 monitor, prefer it easily above a CGA. But am MDA card is a poc. Hercules is a better card. And IINM supports cga games on a 5151???

If it's just the monitor.. You can consider getting an AT SmallWonder or ATI EGA Wonder or similar.
Some models like this have the ability to display CGA graphics in grayscale on an TTL monitor.

Edited.

Thank you for all the elaborate answers!

So to summarize:
- PC VGA cards always scale all sub VGA (< 640x480) resolutions to 480p
- I can always expect the card (S3 Virge 325 in my case) to output at least 640x480
- essentially all monitors do accept this resolution without issues
- I can adjust the aspect ratio so it doesn't appear stretched

Is this correct?

I once had a VGA to HDMI converter in use with this card. It did not convert the POST or other start up messages, only the BIOS or the OS (Win98). In the manual for the converter 640x480 was a supported resolution. If the VGA card is always emitting 640x480, why did it not convert this resolution then?

Dr.Atom wrote on 2024-08-29, 09:47:

- PC VGA cards always scale all sub VGA (< 640x480) resolutions to 480p
- I can always expect the card (S3 Virge 325 in my case) to output at least 640x480

No. In general, resolutions that have less than 350 lines will be line-doubled by the card. So the 200 line modes will be output using 400 lines (twice the number of lines), not 480. 350 line modes will be output unmodified with 350 lines.

I once had a VGA to HDMI converter in use with this card. It did not convert the POST or other start up messages, only the BIOS or the OS (Win98). In the manual for the converter 640x480 was a supported resolution. If the VGA card is always emitting 640x480, why did it not convert this resolution then?

Because the startup text mode is 400 lines, not 480.

clb wrote on 2024-08-28, 20:21:

Also of note, the regular DOS 80-column OS text prompt was 720x400 pixels in size, not scandoubled.

Is a graphics mode 720x400 achievable on a VGA?

Azarien wrote on 2024-08-29, 10:48:

clb wrote on 2024-08-28, 20:21:

Also of note, the regular DOS 80-column OS text prompt was 720x400 pixels in size, not scandoubled.

Is a graphics mode 720x400 achievable on a VGA?

Yes, 720*350, 720*400 and 720*480 are achievable on VGA. You can even go slightly higher by repurposing part of the overscan area as display area. I suspect 752 * 500 or something like that will still have most of the image displayed on a typically adjusted VGA CRT monitor. You won't have fun with that mode on LCD monitors that try to guess the resolution and sample the pixels for digital scaling. You do need to set up the 720*anything graphics modes by manually programming the video hardware. The video BIOS does not provide any means that help you to initialize the 720x400 mode.

Basically, for the horizontal resolution, the VGA card lets you choose between wider pixels, which fill the image at 640 pixels and narrower pixels that fill the image at 720 pixels. Both of these pixel sizes can be doubled to achieve 320 pixels or 360 pixels per scan line. Given suitable SVGA cards, you can set them up to produce even narrower pixels (by using a higher pixel speed), fitting more than 720 pixels in one line - but then you start getting into the resolution limit of typical VGA monitors.

For the vertical resolution, VGA monitors allow 350 lines, 400 lines or 480 lines to vertically fill the "image area". You can also use double-scanning to get 175, 200 or 240 lines of image content displayed on the screen. I've never seen 175 lines double-scanned to 350 in practical use, though.

You can mix and match any horiztonal resolution with any vertical resolution. At 256 colors, the VGA memory and data processing speed is insufficient to keep up with 640/720 pixels, and only the 320/360 pixel modes are available.

jmarsh wrote on 2024-08-29, 10:29:

I once had a VGA to HDMI converter in use with this card. It did not convert the POST or other start up messages, only the BIOS or the OS (Win98). In the manual for the converter 640x480 was a supported resolution. If the VGA card is always emitting 640x480, why did it not convert this resolution then?

Because the startup text mode is 400 lines, not 480.

The DOS boot sequence can be a bit interesting. I have a 80 MHz 486 with an Award BIOS and at the moment an Aska Zymos Poach 51 AA VGA card in it (a Trident 8800CS clone card). I gave the boot sequence a try, and what I see is:

1. Execution is handed off by BIOS to the Option ROM section in the video card BIOS memory. It shows the video card BIOS init in VGA 80x25 (720x400 px) video mode:

2. After video card init, the Award BIOS POST screen comes up. the Award BIOS displays this screen in an EGA 640x350 video mode (this mode is 80x25 characters as well, but each individual character is 8x14 pixels instead of VGA 9x16 pixels):

3. The DOS boot sequence continues back in the same VGA 80x25 (720x400 px) video mode:

4. and DOS will then stay in that video mode:

so displays that cannot handle EGA or VGA modes wouldn't be able to show the boot sequence at all.