Take the binary conversion for given two decimal numbers 257 and 514.
257 => 0100000001
514 => 1000000010
Convert to binary numbers and take first 10 LSB's.
Because all registers in above system are 10-bit registers.
R1 <= 0100000001
R2 <= 1000000010
finally R3 stores sum of the values in R1 and R2 i.e., R3 <= R1 + R2
R3 <= 1100000011