(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? 😀