[derekzhouzhen]

Other has mentioned MMIO. MMIO has several kinds:

1. CPU accessing GPU hw with uncache-able MMIO, such as lower level register access

2. GPU accessing CPU memory with cache-able MMIO, or DMA. such as command and data stream

3. CPU accessing GPU memory with cache-able MMIO, such as textures

They all happen on the bus with different latency and bandwidth.