STMicroelectronics STM32F405 Reference Manual page 1224

Ethernet (ETH): media access control (MAC) with DMA controller
Bit 2 OSF: Operate on second frame
Bit 1 SR: Start/stop receive
Bit 0 Reserved, must be kept at reset value.
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When this bit is set, this bit instructs the DMA to process a second frame of Transmit data
even before status for first frame is obtained.
When this bit is set, the receive process is placed in the Running state. The DMA attempts to
acquire the descriptor from the receive list and processes incoming frames. Descriptor
acquisition is attempted from the current position in the list, which is the address set by the
DMA ETH_DMARDLAR register or the position retained when the receive process was
previously stopped. If no descriptor is owned by the DMA, reception is suspended and the
receive buffer unavailable bit (ETH_DMASR [7]) is set. The Start Receive command is
effective only when reception has stopped. If the command was issued before setting the
DMA ETH_DMARDLAR register, the DMA behavior is unpredictable.
When this bit is cleared, RxDMA operation is stopped after the transfer of the current frame.
The next descriptor position in the receive list is saved and becomes the current position
when the receive process is restarted. The Stop Receive command is effective only when
the Receive process is in either the Running (waiting for receive packet) or the Suspended
state.
DocID018909 Rev 11
RM0090