What about PCjr (Tandy 1000) compatibility?
What about PCjr (Tandy 1000) compatibility?
Since this topic is letting us go wild with our creativity anyway, here's a crazy idea:
How about adding support for PCjr graphics modes such as 320x200x16 and 640x200x4 to such a card? I know that the Video Gate Array was based on shared video, but it's mapped to the standard B8000 area. Putting 128KB of video memory on said fantasy/alternate-universe CGA card would be cheating, but what if this card would have a gimmick where it would "sit between" the memory sockets of the first 128KB of on-board memory and the RAM chips? So this card would go into an 8-bit ISA slot, have empty memory sockets of the same type that go into a standard IBM PC that could take up to 128KB of regular IBM PC RAM. It would also have a ribbon cable attached to the card which would then connect to the RAM sockets of the motherboard.
The card would come without RAM when bought, keeping it cheap, and people installing such a card would be instructed to remove the first 128KB of memory from the motherboard, place the removed RAM chips in the sockets on the card, and then attach the ribbon cable coming out of the card into the vacated RAM sockets on the motherboard.
For 8086-based clones, such as the Olivetti M24 and AT&T 6300, a special version could be developed that would support a 16-bit memory bus, and would fit in the proprietary 16-bit expansion slots on those models.
And while we're designing such a CGA+ card anyway, we might as well throw in three-voice audio compatibility, which would install in a similar way: people would have to disconnect the wires from the internal speakers, connect them to input pins on the sound card, and then either connect the output of the card to the existing internal speaker, or use an external audio output on the back of such a card. That would pretty much complete after-market game-mode PCjr and Tandy 1000 compatibility.
Would this be too crazy an idea? And would this be "cheating", in the sense that such a card would have to be feasable with the same technology and hardware budget available to IBM back when they designed the original CGA card? Shipping the card without memory and letting people reuse part of the RAM on their motherboards would keep the card cheap enough to remain within that budget, right?
Or would this be too complicated to implement, or are there perhaps other technical hurdles that I've missed?