The Asus AM1-IA motherboard I have unfortunately doesn’t have a BIOS option to enable the IOMMU, despite the fact AM1 socket processors theoretically support it.
Under linux you get the following dmesg output:
Oct 17 16:36:19 tobermory kernel: AGP: Checking aperture...
Oct 17 16:36:19 tobermory kernel: AGP: No AGP bridge found
Oct 17 16:36:19 tobermory kernel: AGP: Node 0: aperture [bus addr 0x00000000-0x01ffffff] (32MB)
Oct 17 16:36:19 tobermory kernel: AGP: Your BIOS doesn't leave a aperture memory hole
Oct 17 16:36:19 tobermory kernel: AGP: Please enable the IOMMU option in the BIOS setup
Oct 17 16:36:19 tobermory kernel: AGP: This costs you 64MB of RAM
Oct 17 16:36:19 tobermory kernel: AGP: Mapping aperture over RAM [mem 0x80000000-0x83ffffff] (65536KB)
Oct 17 16:36:19 tobermory kernel: PM: Registered nosave memory: [mem 0x80000000-0x83ffffff]
To prevent, pass the kernel parameter
iommu=soft