CLASS #4: Register and Immediate addressing

    100 rrr p00 sss        p=0 => rrr <-- sss   ; Register addressing     { ldr r1, r2   }
                           p=1 => rrr <-- value ; Immediate (sss ignored) { ldi r, value }

       In the case of Immediate addressing (p=1), the operand (value) is given
       in the word following the opcode.