DDX
For each type of graphics card there is an XFree86 2D (or DDX) driver which does initialization, manages the display and performs 2D rendering. XFree86 4.0 introduced a new device driver interface called XAA which should allow XFree86 drivers to be backward compatible with future versions of the X server.
Each 2D driver has a bit of code to bootstrap the 3D / DRI features.
Where does the DDX driver resides?
The XFree86 drivers usually reside in the /usr/X11R6/lib/modules/drivers/ directory with names of the form *_drv.o.
The XFree86 drivers source code resides in xc/programs/Xserver/hw/xfree86/drivers/ usually in a file *_dri.[ch].