Behavior observed by poking at an ET4000AX:

I/O ports are decoded from the full 16 bits. There is no aliasing of VGA I/O ports.

However, there is an I/O port at 0x46E8 (documented by VGADOC) where bit 3 is a
'VGA enable' bit. When you write to it, you enable/disable decoding of VGA ports.
I noticed that this particular I/O port is the only one offered by the ET4000AX
where a full 16-bit decode is NOT used, but instead only decodes the lower 12 bits.
That means that in addition to the documented port 0x46E8, you can also write this
port through aliases 0x06E8, 0x16E8, 0x26E8, 0x36E8 up through 0xF6E8.

True to the documentation, clearing bit 3 of port 0x46E8 disables all VGA I/O ports
offered by the ET4000AX chipset.

