The SD interface controller maps the internal signals to the external SD interface and vice versa. Based on the bus width (1, 4, or 8) the internal signals are driven out appropriately.
In the case of a default speed (DS) mode, the outputs are driven on the negative edge of the SD_CLK.
The inputs are latched on the RX_CLK (looped back or tuned clock) and output to the receive control unit for further processing.