Hi, I'm wondering why DOS-based systems, especially Windows 98, can't use more than one CPU core or thread at a time. I know it's due to limitations in the DOS-based kernel, but the issue doesn't end when I find out there was a dual GPU, which was fully optimized for Windows 98 and Me (on NT-based systems like XP, it was only detected as one), which was the ATI Rage Fury Maxx.
From what I've seen in videos and screenshots of this dual GPU, you can see two display drivers listed in the Device Manager, as is the case with more modern dual GPUs like the GeForce Titan, the GTX 590, or the 690. How did ATI overcome this limitation?

The GPU is the chip on the card, not the video card itself. Each video card is a bus-master device so each GPU can directly access main memory while running its own code, just like the CPU. Each display driver gets a slice of CPU time to refill the GPU instruction and data pipelines so it can keep running concurrent to other GPUs and the CPU.

Later NT versions have the equivalent of drivers for each CPU/core/hyperthread built into the OS. Windows 9x and earlier do not so can only make use of one CPU core.

ATI didn't fully overcome the one-core CPU bottleneck--the display drivers still use it.  But by creating powerful multiple GPU/core subsystems with lots of private, higher performance RAM, they were able to off-load most of the complex graphics rendering from the single, shared CPU.

 

>Why doesn't Windows 98 work with a multi-core CPU?

Because precisely Windows 98 apeared too early to get this feature and in its descendant (ME) the creator of both systems refused to add this feature in favor of NT line OSes.