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342 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
PCI Unit
9.4.1.3 DMA Descriptor
Each descriptor occupies four 32-bit Dwords and is aligned on a 16-byte boundary. The DMA
channels read the descriptors from local SRAM into the four DMA working registers once the
control register has been set to initiate the transaction. This control must be set explicitly. This
starts the DMA transfer. The register names for the DMA channels are listed in Figure 127.
After a descriptor is processed, the next descriptor is loaded in the working registers. This process
repeats until the chain of descriptors is terminated (i.e., the End of Chain bit is set). See Table 126.
Figure 127. DMA Descriptor Reads
A9774-01
Local SRAM
4
3
1
2
Last
Descriptor
Next
Descriptor
Prior
Descriptor
Current
Descriptor
Working Register
Control Register
Byte Count Register CHAN_X_BYTE_COUNT
PCI Address Register CHAN_X_PCI_ADDR
DRAM Address Register CHAN_X_DRAM_ADDR
Descriptor Pointer Register CHAN_S_DESC_PTR
Channel Register Name (X can be 1, 2, or 3)DMA Channel Register
Control Register CHAN_X_CONTROL
Channel Register Name (X can be 1, 2, or 3)DMA Channel Register
Table 126. DMA Descriptor Format
Offset from Descriptor Pointer Description
0x0 Byte Count
0x4 PCI Address
0x8 DRAM Address
0xC Next Descriptor Address