I think it's down to the keyboard controller, which may be separate from the chipset. Some integrate the USB into basic BIOS i/o that works in DOS some are just basic parts that do what they did ever since the 286 and not a jot more. It can be the sign of a sick KBC if any keyboard works in windows and not DOS.

I always thought it was the BIOS that had code that would translate standard keyboard BIOS calls into USB and back, but I must admit I never thought about games that access the keyboard I/O ports directly. I guess they work with USB keyboards too? Maybe that is down to the chipset as well.

USB keyboards (and mice) have a 'boot protocol' which is used for this emulation, and it's a different USB protocol than is normally used when full USB support is available. So I guess this simpler protocol is to allow a chipset to easily provide legacy keyboard ports without needing a full USB stack.

Well yes, the BIOS needs to support it too, but won't without hardware that kinda links the 32 bit stuff with the legacy 16 bit stuff.

Actually If BIOS has 'USB Legacy support' option, it creates virtual PS/2 keyboard/mice and translates all inputs from the real USB devices to these virtual PS/2 ones using SMM. Of course, for DOS those emulated PS/2 devices looks like real 😀

If BIOS does not has 'USB Legacy support' option, you can use TSR drivers for DOS (link ). But they are abandoned in beta-stage and only support some types of usb controllers. And of couse they have compability issues with some applications (yeah, it's DOS baby! 😁).