Talk:IBM Multicolor Graphics Adapter

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FunkyMonkey: I'm confused why you say there's no "official" descriptions of MCGA; they're all over the place. It is *not* just a VGA with 64K of RAM; if it were, it could do stuff like 320x200x16 or 640x200x16 which obviously it can't. It doesn't support tweak modes, it doesn't have all of VGA's registers... please don't alter the MCGA Wikipedia entry based on conjecture. Instead, pick up a used copy of Ferraro's EGA and VGA book.

256-color mode was officially introduced on both the MCGA (IBM PS/2 Model 25 and Model 30) and the VGA (Model 50 and others) simultaneously in 1987. -- Trixter

I've been puzzling over the MCGA for some time, I can find no 'official' descriptions of it.

I believe that in truth it's maybe a VGA with only 64K of memory - that would place it in between the EGA and VGA more accurately. My reasons for coming to this conclusion are that the 256 colour mode was most prominently introduced on the VGA, after the EGA. It's generally accepted that the VGA has 256K of Memory, but according to my IBM VGA technical reference the VGA has a register bit indicating whether 64K or 256K memory is installed - however during the rest of the documentation, 256K is always assumed.

I'll leave this one open for a while to see if anyone has any ideas, or information that contradicts this, otherwise i'll change the page to reflect this in a little while. --FunkyMonkey.

According to an old edition of "Upgrading and Repairing PCs", MCGA was compatible with CGA analog monitors, had 31.5 kHz scan rate, 64K multi-port RAM, a palette of 262,144 colors, and used a 15 pin D shell connector. Top resolution mode was 320 * 200 in 256 colors at 70 Hz, or 640*480 in 2 colors at 60 Hz. Mueller says (in 1992) that the Model 25 and Model 30 that had this mode were discontinued, and that no-one ever cloned this display adapater because VGA was so much better at little extra cost.

The article erred when it said the MCGA was the first 256-color dislay standard; that goes to the obscure 1984 PGA adapter, which I've stuck in with the Enhanced Graphics Adapter with which it was roughly contemperaneous.

And nowadays laptops look odd if they accidentally boot up in as low as 640*480 mode....what progress in 20 years. --Wtshymanski 21:22, 23 May 2005 (UTC)

Well - the 15 pin D-shell, analogue output, 31.5Khz scan rate and 262,144 colour palette are ALL features of the VGA,not the CGA, which has a digital output (and possibly an analogue composite output). The top resolution of 640x480 would also fit in nicely with the VGA's 25/28Mhz of bandwidth - further supporting my theory that the MCGA is in fact, simply a VGA with only 64K of RAM installed, or at least, a very close cousin of the VGA. A huge thanks for the info, great to hear from people interested in these arcane subjects :D -- FunkyMonkey

Hmmm - the plot thickens - the last contributor noted that MCGA was *not* EGA compatible, so that maybe blows my VGA theory out of the window. If we go back to the the "close cousin" of the VGA theory though, maybe the MCGA is like a 64K VGA but lacking the features present in the EGA/VGA's attribute controller such as the internal 64 colour palette. This is the only reason I could think the EGA games wouldnt work... -- Funkymonkey.

Text mode on an MCGA is 720x400, 70 hertz vertical refresh rate, just like a VGA. You can verify this by changing text-mode fonts and observing the 9x16 character cell.

As far as I can tell from poking around MCGA registers, it is neither a subset of VGA nor an extension of EGA. One significantly missing feature is bit-plane addressing. You can still change text-mode fonts, but you have to change them through the BIOS, not through bit plane 2--I never disassembled the BIOS to find out where the text-mode fonts live in video memory.

Like the MCGA, the PS/2 Model 25 and 30 have a unique BIOS not found in another computer. The display BIOS isn't similar to an EGA or PS/2 VGA.

I think it's worth noting that the CGA compatible modes are actually 320x400 modes with scan line doubling from the video memory, just like the MCGA and VGA's 320x200 256 colour mode. CGA features like switchable colour palettes (which work on an EGA as well) don't work on an MCGA or VGA.

-- Joshua 17:32, 9 December 2005 (UTC)