     decrx     headr
     *                                                               )0D
     * Decrement x)01 by )02 and store it in )0C                     )0D
     * Decrement x)01 by )02 and store it in X)01                 )0t)0D
     * The register label is assumed to be X)01.                  )0t)0D
     *                                                               )0D
     )00       sbr  )0C,16000-)02&X)01
     )00       sbr  X)01,16000-)02&X)01                              )0T
