In the SI format, the contents of the general register designated by the B1 field are added to the contents of the D1 field to form the first-operand address. In the S and RS format, the contents of the general register designated by the B2 field are added to the contents of the D2 field to form the second-operand address. In the RX format, the contents of the general registers designated by the X2 and B2 fields are added to the contents of the D2 field to form the second-operand address.
Five Basic Instruction Formats