SCL      RD, immediate, R1 or memory, R2 or memory/immediate
         RD, immediate, R1 or memory, R2 or memory/immediate, cmd
         RD, immediate, R1 or memory, R2 or memory/immediate, cmd, dt_flags
         RD, immediate, R1 or memory, R2 or memory/immediate, cmd, dt_flags, d_sign-1