HD4400 4MB VRAM

[RehabMan]

My port is pure digital output(DVI-D) then I use the cable to convert digital signal to analog signal

If your port on the motherboard was only DVI-D, then it would not work with a VGA monitor even in Windows (at least not without an "active" adapter)

Most DVI ports are DVI-I (which is DVI-D plus DVI-A)