Direct Memory Access (DMA)
This is the most complex of the data-transfer systems as both the host and the slave can access each others memory in any way they chose. As most !CS or !CprE people can quickly see, that only leads to more problems. So, DMA programming is the most complex of the three normal types of memory access. With DMA, an external chip/processor (let's be honest, a 3D processor counts as an external processor in its own right) can see anything in out memory space and we can see anything in its memory space -- barring limitations from things like the GART. With DMA, a typical interaction with an external chip/ processor is "here's an address at which you can find a complex data structure in my memory space in which you can find the commands and data to describe the operation which I wish you to perform. Have fun. Oh, tell me when you're done." DMA will normally allow the highest level of performance, but it does require some compromises that may sacrifice some latency for bandwidth.