AGP mapping problem

[cckuo]

Dear X friends:
I intend to enable AGP and map it to the virtual memory such that to write
HW commands. 
I reference the enable sequence wrote in R128_dri.c, which looks like
1. drmAgpAcquire (make sure AGP is available)
2. drmAgpEnable (give the mode,like 2x, 4x)
3. drmAgpAlloc
4. drmAgpBind
5. drmAddMap
6. drmMap 
In the final step, drmMap, the last parameter we pass to Xserver is
drmAddressPtr. 
Is this variable the virtual address OS map to the AGP physical address?
If it is, each time I try to write some value to this pointer, the Xserver
will report signal 11, and shutdown.
Is there some parameters I need to set such that I can get a correct virtual
address to write or I should use different way to get mapping Virtual
address?

Thanks a lot!
Pierre