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IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design
Application Note 39
Modified on: 3/20/02,
Both descriptors and buffers are stored in arrays. The array index is used to associate a unique
DRAM Data Buffer with each SRAM Descriptor:
4.4.1 SRAM Buffer Descriptor Format
This buffer descriptor format is used throughout the design, except when a descriptor is enqueued
onto a packet_queue for Ethernet transmit.
Figure 27. SRAM Descriptor to DRAM Buffer Mapping
A9783-01
SRAM
SRAM
descriptor [i+1]
descriptor [i]
descriptor [i+2]
data buffer [i+1]
data buffer [i]
data buffer [i+2]
Figure 28. Buffer Descriptor Format for ATM Transmit Destination Port
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9876543210
0Z Next BD
1 Last Quad X
2 Queue Index Start Byte Offset Y
3 ATM Header
Entry Description
Z Unused - will be overwritten upon enqueue/dequeue address updates
Next BD 32-bit SRAM address of the next buffer descriptor in the same queue
Last Quad Offset of the last quadword in the buffer that contains data
X Unused - will be erased every time LAST_QUAD is updated, Rx any cell
Queue Index Index of the queue where this descriptor came from
Start Byte Offset Offset of the first byte of data to be transmitted
Y Unused - will be erased every time Start byte offset is updated, Rx first cell -- Tx any cell
ATM Header ATM Header (w/o HEC) to be attached to each cell of the PDU in the buffer