Introduction: Three Types Of Traffic Flow


Hypertransport defines three types of traffic: Programmed I/O (PIO), Direct Memory Access (DMA), and Peer-to-Peer. Figure 6-1 on page 121 depicts the three types of traffic.

  1. Programmed I/O traffic originates at the host bridge on behalf of the CPU and targets I/O or Memory Mapped I/O in one of the peripherals. These types of transactions often are generated by CPU to set up peripherals for bus master activity, check status, program configuration space, etc.

  2. DMA traffic originates at a bus master peripheral and typically targets main memory. This traffic is used so that the CPU may be off-loaded from the burden of moving large amounts of data to and from the I/O subsystem. Generally, the CPU uses a few PIO instructions to program the peripheral device with information about a required DMA transfer (transfer size, target address in memory, read or write, etc.), then performs some other task while the DMA transfer is carried out. When the transfer is complete, the DMA device may generate an interrupt message to inform the CPU.

  3. Peer-to-Peer traffic is generated by an interior node and targets another interior node. In HyperTransport, direct peer-to-peer traffic is not allowed. As indicated in Figure 6-1 on page 121, the request is issued upstream and must travel to the host bridge. The host bridge examines the address and determines whether the request should be reflected downstream. If the request is non-posted , the response will similarly travel from the target back up to the host bridge and then be reissued to the original requester.

Figure 6-1. PIO, DMA, And Peer-to-Peer Traffic

graphics/06fig01.jpg



HyperTransport System Architecture
HyperTransportв„ў System Architecture
ISBN: 0321168453
EAN: 2147483647
Year: 2003
Pages: 182

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