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The BRAM_Addr signal is a 32-bit value interpreted as a byte address by the bram_block. The bram_block is a 64-bit memory which requires the BRAM_Addr signal be 8-byte aligned (i.e. its 3 LSB are always 0).

The Tx PHY logic generates an address for the index of the current bit. In order to retrieve the memory word which contains the current bit, the PHY's bit index must be translated into a 8-byte aligned BRAM word address. The BRAM word address for a given bit index X is (floor(X / 64)). Division by 64 is equivalent to right-shift by 6, implemented here as slicing out the 9MSB of the 15-bit bit index value.