The invention introduces an apparatus for segmenting a data stream, installed in a physical layer, to include a host interface, a data register and a boundary detector. The data register is arranged to operably store data received from the host side through the host interface. The boundary detector is arranged to operably detect the content of the data register. When the data register includes a boundary-lock pattern or a special symbol, the boundary detector outputs a starting address that the boundary-lock pattern or the special symbol is stored in the data register to an offset register to update a value stored in the offset register, thereby enabling a stream splitter to divide data bits of the data register according to the updated value of the offset register.