I got a really good price for an Aten CS1734A KVM switch, that can use both USB and PS/2 cables. The USB cable does not work with bios legacy USB, but the PS/2 cable does. The KVM emulates a keyboard and mouse, and then map the inputs from the actual keyboard and mouse through two dedicated USB ports. It works fine with the keyboard, but the mouse emulation is pretty bad. Without the emulation, the mouse works perfectly, but only in operating systems that support USB HIDs like for instance windows 98 (has anyone tried using the win98 usb hid driver in dos 6 to see if it works?). I only have one dos 6 pc that relies solely on legacy USB, so for me its ok to have an extra mouse just for that one computer.
I have noticed that not all USB mice connected to the KVM work equally bad with emulation enabled. I have an older optical mouse with a red diode that is completely useless, and a newer laser mouse that works somewhat better, and i started thinking it could be because of the older mouse drawing more power from the USB port than the newer mouse. I tried powering the KVM with a 2A USB charger (the KVM is powered by the USB and PS/2 ports of the host computers) but it didnt help. I will try connect a powered USB hub in between the emulation port an the mouse, so that the power draw from the KVM becomes minimal, to see if it works better.