Abstract: An add with circular mask operation is executed in a RISC processor which includes a coprocessor having a register for storing a circular mask value. A circular mask instruction to the coprocessor includes a value in an immediate field and identifies a general register (RS), and a destination register (RT). The coprocessor operates on the value stored in the general register with the value in the immediate field and then masks the results using the circular mask value. The results are then stored in the destination register. The operation includes sign-extending the immediate field before adding to the contents of the general register to provide a sum, and the sum is then masked with the circular mask value.