RDEC     RD, R1 or memory, R2 or memory/immediate
         RD, R1 or memory, R2 or memory/immediate, dt_flags
         RD, R1 or memory, R2 or memory/immediate, dt_flags, d_sign-1
         RD, R1 or memory, R2 or memory/immediate, dt_flags, d_sign-1, R3 or shift_immediate