pci-dma disables iommu on nforce4 motherboards?

[Ed Sweetman]

I have an asus A8N-E motherboard and recieve the following message on 
boot. 

PCI-DMA: Disabling IOMMU.

I have no issues with anything not functioning.  I guess i'm just 
curious as to why this is done and if i'm missing out on any sort of 
performance gain by not using the iommu.   I have less than 4GB of ram, 
would that be why it's disabled (which is why i think it is)?  
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