Abstract: This invention relates to a right justified mask transfer apparatus which may be effectively utilized in a digital data computer. A right justified mask transfer register is provided which holds the result of a logical bit-by-bit ANDing of a multi-bit data word with a multi-bit mask work such that only those bit positions for which the mask word are of a predetermined binary significance are collected contiguously in the mask transfer register, all other bit positions being ignored. Further operations on intermediate digital data computations may be more easily performed by utilizing the right justified mask transfer function to select certain data bits or fields.