When I choose a 2GB DDR2 memory and set the base address to 0x8000 0000 (halfway through the entire address range), the tcl script errors out with the following:
ERROR:MDT - issued from TCL procedure "update_mpmc_highaddr" line 48
DDR2_SDRAM (mpmc) -
Auto-computed high address overflows the 32-bit address space. Setting base address to 0x00000000.
In general, this error would occur when the MPMC address range covers the top of the address map (i.e., from anything to 0xFFFF_FFFF).
You can safely ignore this error.
This issue has been fixed in MPMC v4.01.a and later versions. It was first released in EDK 10.1 Service Pack 1.