Triple-Speed Ethernet Intel® FPGA IP User Guide

ID 683402
Date 11/25/2022
Public

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4.1.4.9. IP Payload Alignment

The network stack makes frequent use of the IP addresses stored in Ethernet frames. When you turn on the Align packet headers to 32-bit boundaries option, the MAC function aligns the IP payload on a 32-bit boundary by adding two bytes to the beginning of Ethernet frames. The padding of Ethernet frames are determined by the registers tx_cmd_stat and rx_cmd_stat on transmit and receive, respectively.
Table 25.  32-Bit Interface Data Structure — Non-IP Aligned Ethernet Frame
Bits
31...24 23...16 15...8 7...0
Byte 0 Byte 1 Byte 2 Byte 3
Byte 4 Byte 5 Byte 6 Byte 7
Table 26.  32-Bit Interface Data Structure — IP Aligned Ethernet Frame
Bits
31...24 23...16 15...8 7...0
padded with zeros Byte 0 Byte 1
Byte 2 Byte 3 Byte 4 Byte 5