Talk:Video Graphics Array

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Contents 1 Memory base 2 Removed 640x400 Mode-X and Direct-X reference 3 VGA 256x177? 4 Meaning of VGA

[edit] Memory base

From the article: the video memory for color mode is mapped at 0xb8000-0xbffff.

I thought VGA graphics memory started at 0xa0000? At least, in linear (320x200x256) mode, where each byte was one pixel? -- pne 05:07, 26 Aug 2004 (UTC)

  Answer:  In graphics mode yes it's 0xa0000.  Colour text mode is "0xb8000".  -- Funkymonkey.

What about the VESA standard for successors to VGA?

  Feel free to add your own info :)  -- Funkymonkey.

0xb8000-0xbfff is also used for the old CGA color modes (320x200x4, 640x480x2) and text mode. Everything else (320x200x256, 640x480x16, and the EGA 16-color modes) use 0xa0000-0xaffff. 0xb0000-0xb7fff isn't usually touched by VGAs since it's the MDA text buffer. The VESA extensions to VGA are described over in Super VGA. -lee 17:17, 21 Sep 2004 (UTC)

  0xb0000-0xb7fff is a perfectly valid address space for the VGA when operating in a Mono text mode (Mode 7).  
  -- Funkymonkey

Vector Graphics Array

[edit] Removed 640x400 Mode-X and Direct-X reference

I removed the reference to 800x600 and 640x400 modes, as I'm pretty sure they're not possible using standard VGA hardware. 800x600 maybe at a low refresh rate? Remember the distinction between a clone VGA and a "Super VGA" is blurred, some clone VGA's such as Oak's OTI037 256K VGA were capable of 800x600 I remember. However, this page is about the true blue IBM original.

The main reason these modes should be near impossible (especially 640x400 in 256 colours) on standard VGA hardware is that the video bandwidth (28Mhz max) is too low. Horizontal Scan Rate would be unacceptably low. I'd love to be proved wrong however - if someone can demostrate the CRTC settings for a 640x400 256 colour, or 800x600 mode that would run on an IBM VGA with multisync monitor i'd be interested to see it.

I also removed the reference to the 'Direct-X' term double buffering. Double Buffering has long been used as a term before the introduction of Direct-X. -- Funkymonkey

May 8, 2005 - Hi Funkymonkey! There was an old MS-DOS program "FRACTINT", which claimed to support output resolutions up to 800x600 (16-color) on a true "IBM VGA adapter." Years ago, I fooled around with the program, but on an SVGA adapter, so I can't verify the program's claims. I do remember "ModeX" allowed up to 360x480 (60Hz) without resorting to outrageous refresh-rates -- a handful of MS-DOS games used this mode (my favorite was "Bananoid", a shareware clone of Arkanoid.) http://spanky.triumf.ca/www/fractint/hardware_modes.html#video_notes_anchor

June 23rd, 2005. I read that link and it seems you're right about the 800x600 mode - tres cool, and a good find! We should integrate the information from that link back into the VGA page, I think i'll do that! -- Funkymonkey.

I can provide working programs that display 800x600 in 16 colors and 640x400 in 256 colors on a STOCK VGA so I know it is possible. The only drawback is that most monitors couldn't display it (it requires a very low refresh rate). If those were taken out, I'm going to put them back in. Trixter 21:38, 12 January 2006 (UTC)

Cool - I'd love to see those programs, if you can make them available. What monitors have you found are able to sync to these low refresh rates? Thanks. Funkymonkey

I'd have to find the code :-) But fractint is one of the 800x600@16 programs so just grab that and try it in DOS fullscreen. As for monitors, not many unfortunately. Trixter 23:59, 16 January 2006 (UTC)

I'm willing to accept that 800x600 is possible, and it has been show (Fractint as you say) but 640x400 in 256 colours seems unlikely due to the very low horizontal frequency that would involved. In 256 colour modes, the VGA is operating at half the horizontal clock speed as two normal clocks elapse for each pixel. Can we leave out the reference to 640x400 in 256 colours until there is some proof of it's existance? --Funkymonkey.

I guess I'm going to have to find that code then. But unless you have a low-scanning monitor (one that can do 15KHz horizontal) you probably won't get it to work. It *is* possible, I've done it, but just like 800x600x16 it displays on about 1% of monitors out there.

But what's wrong with the paragraph right under it? Does it not explain the limitations of trying to use such extreme modes? This was a big deal back then, I remember a lot of discussion around getting ColoRIX to use additional tweaked modes... Trixter 21:53, 25 January 2006 (UTC)

Leave it in if you like then, I just think it's nice if we have some evidence to backup the mode's described as possible. --Funkymonkey.

[edit] VGA 256x177?

Could be possible that VGA can display 256x177?

I found that when playing Rastan on Dosbox. Or pearps it's EGA tweak.

You're correct. Rastan tweaks a 256x177 mode. The 256 pixel columns across is not only easy but common; the 177 lines is the unconventional part.

256 pixel columns was a huge boon to game programmers -- it meant you could treat screen coordinates as going from 0 to 255 across, which meant your "X" variable could fit into a single byte-size register. Some games used this and put a status display up from pixel columns 256-319, but a few games, like Rastan, actually tweaked a 256-column mode. Trixter 23:10, 10 January 2006 (UTC)

Thanks. I played Rastan on my old computer with a TV plugged. The screen displays odd effects when playing Rastan.

That was CGA composite output, which displayed odd artifacts if the game didn't specifically support it. See the wikipedia article on CGA for more info. Trixter 21:39, 12 January 2006 (UTC)

My old computer does not have a CGA composite output. I used a device to connect the TV via VGA output. Peharps the device was not designed to use 256x177 mode.

It's ever worst when connecting the tv to my newest computer connected via 3d card and playing Rastan. Everything's flickering.

[edit] Meaning of VGA

It is my understanding that VGA was an important landmark in that, for the first time, standard IBM PC color computer monitors came to have the same resolution as North American TV broadcasts (NTSC standard); thence the name "Video Graphics Array", or an array of pixels matching the resolution of standard video.

As written the article named "NTSC", color TV broadcasts have 486 "viewable" horizontal scanning lines per frame which closely matches VGA's 480 number. As for the 640 number it surely must match the equivalent resolution if the scanning were done with vertical lines instead of with horizontal lines, but I'm unsure about this.

I don't know were to add this comment without disturbing the balance of this article. Perhaps if the original author sees this note he/she may add it appropriately.

IIRC, the 486 comes from some digital video format (DV, D1?) while broadcast NTSC has indeed 480 lines (but often more than 640 pixels per line when transmitted digitally). Christoph Päper 22:04, 1 June 2006 (UTC)