These measurements are made using one or more PCI-E exerciser cards developed internally at Sun and controlled using a custom Solaris driver. Each card contains several hardware engines that can be programmed to perform Direct Memory Access (DMA) operations at very high rates. The exerciser hardware can also measure round-trip latency for DMA reads, the time it takes for a DMA read request made by the exerciser to return data from host memory. By programming the DMA engine appropriately, the operation is performed exclusively by hardware, so the reported latency does not include software/driver overheads.