VGA emulation in BOCHS

I am looking at how BOCHS emulates VGA. it seems to be that VGA memory
is within a fixed memory range and different processors can write to
the memory using PIO or MMIO.  when the countDownEvent is called. The
current states in the VGA buffer is updated to the X window through
xcblib APIs. I have 2 questions.
1. what sets up the fixed memory region for VGA ? The BIOS ?
2. Is the update to x windows only invoked from the vga update
function which is called by countDownEvent ?

Thank you,
Xin

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