The transmitter checksum offload is only available when the full store and forward mode is enabled. This is because the complete frame to be transmitted must be read into the packet buffer memory before the checksum can be calculated and written back into the headers at the beginning of the frame.
Transmitter checksum offload is enabled by setting bit  in the DMA configuration register. When enabled, it monitors the frame as it is written into the transmitter packet buffer memory to automatically detect the protocol of the frame. Protocol support is identical to the receiver checksum offload.
For transmit checksum generation and substitution to occur, the protocol of the frame must be recognized and the frame must be provided without the FCS field, by ensuring that bit  of the transmit descriptor word  is clear. If the frame data already had the FCS field, it would be corrupted by the substitution of the new checksum fields.
If these conditions are met, the transmit checksum offload engine calculates the IP, TCP, and UDP checksums as appropriate. When the full packet is completely written into packet buffer memory, the checksums are valid and the relevant status buffer locations are updated for the new checksum fields as per standard IP/TCP and UDP packet structures.
If the transmitter checksum engine is prevented from generating the relevant checksums, bits [22:20] of the transmitter DMA writeback status are updated to identify the reason for the error. The frame is still transmitted, but without the checksum substitution. Typically the reason that the substitution does not occur is that the protocol is not recognized.