Which CGA/MDA combo card is this program for? (ST100A.COM)

Discussion about old graphics cards, monitors and video related things.

Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby VileRancour » 2019-7-10 @ 08:43

(Cross-posting from VCF):

I've finally started dumping a rather sizable pile of floppies, which date back from my family's first no-name XT clone (~'87 or so). The first batch turned up a copy of the TSR attached here, which is related to the video adapter.

One thing I don't really know is what video adapter we actually had in there, so that's what I'm hoping to find out. I had a look into what the program actually does, so my IDA-assisted disassembly is included too.

What I remember:

  • The card was CGA-compatible (at the hardware level, since it ran the usual suspects like Digger and Round 42)
  • The monitor was monochrome, but displayed true 200-line CGA video (in shades of green)
  • 80-column text mode was CGA-like by default (200 scanlines, 8x8 characters)
  • However, the card could *also* display sharp MDA-style text on the same monitor (80x25 text only; no Hercules graphics or anything)
  • "ST100A" was the program that let you switch between the CGA-like and MDA-like text display.

What ST100A.COM actually appears to do:

  • Writes 40h to port 3DDh
  • Sets up an alternate Video Parameter Table, points INT 1Dh at it, then sets video mode 2 (B&W 80x25 text)
  • Installs its own handlers for INT 09h and INT 10h (keyboard and video), then terminates & stays resident
  • You can then hit Alt+[, Alt+], or Alt+/ to control the behavior of text mode.

Looks like "normal" CGA text mode requires sending 00h to port 3DDh, restoring the default Video Parameter Table, then setting mode 2 or 3. Writing 40h to that port (and setting up the alternate VPT) gets you the MDA-type mode instead.
If that's right, then Alt+[ sets up CGA text, Alt+] sets up MDA text, and Alt+/ flips between the two. (The keyboard handler also watches for Ctrl+Alt+Del, and sets up CGA text again before the warm boot.)

There's some other stuff in there too, like cursor handling; but it seems like port 3DDh may be the key here, since it's non-standard.

It's a long shot, but given all that - does anyone have a clue what hardware this was meant for, exactly? :)
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Re: Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby Benedikt » 2019-7-14 @ 17:29

If it writes 40h to port 3ddh for this purpose, that rules out Plantronics ColorPlus compatibility.
Bit 6 of port 3ddh would control page swapping of the red/green and blue/intensity pages on such cards.

Your card can therefore not be based on one of these chip sets:
  • Paradise PVC4
  • ATI CW16800-A
  • ATI CW16800-B
  • ATI 18700
I don't think that a manufacturer would have broken backwards compatibility by including the relatively obscure Plantronics modes, so it's presumably neither an ATI card nor a Paradise card.
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Re: Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby VileRancour » 2019-7-15 @ 09:51

Yeah, I figured as much (the only relevant web result that mentioned port 3DDh was John Elliott's Plantronics ColorPlus page). :) Thanks for the info on specific chipsets, in any case.

I guess the only way to find out for sure is through software/documentation that mentions the specific card... which is quite a long shot.
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Re: Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby Scali » 2019-7-15 @ 09:56

ST, would that be a reference to STMicroelectronics?
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Re: Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby Grzyb » 2019-7-15 @ 14:08

I would rather associate the "ST" with STB, they had various graphics cards in the era, see eg. viewtopic.php?f=63&t=66871
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Re: Which CGA/MDA combo card is this program for? (ST100A.COM)

Postby VileRancour » 2019-7-15 @ 17:23

The only STB cards from that time-frame I'm aware of are the Graphix Plus I/II (CGA+Herc workalikes with extensions) and Chauffeur (extended mono graphics)... doesn't sound like either of those.

Curiously another diskette turned up some utilities that came with a Tecmar Graphics Master, but that one doesn't fit either - more advanced than what we had, and uses a different port. (The data on it tells me it was a work disk created by someone before we had that PC, anyway...)

BTW: it's worth mentioning that the "mystery" card apparently had no onboard BIOS extension, and identified itself as CGA (according to the program it sets up the MDA-like display for modes 2 and 3, not mode 7). As far as I can remember, it still used CGA-like attributes in that mode.
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