MCR指令叫做主控指令，大约说的是有这么一个MCR的栈区。在这个栈区内的操作不仅受分步条件控制，还可以用MCR的ON或OFF来进行总控制！如：
STL Explanation
MCRA //Activate MCR area.
A I 1.0
MCR( //Save RLO in MCR stack, open MCR area. MCR = "on" when RLO=1 (I?.0 ="1"); MCR = "off" when RLO=0 (I 1.0 ="0")
A I 4.0
= Q 8.0 //If MCR = "off", then Q 8.0 is set to "0" regardless of I 4.0.
L MW20
T QW10 //If MCR = "off", then "0" is transferred to QW10.
)MCR //End MCR area.
MCRD //Deactivate MCR area.
A I 1.1
= Q 8.1 //These instructions are outside of the MCR area and are not dependent upon the MCR bit.
在上例中，用I 1.0来作为MCR取激活的条件，既将A I 1.0逻辑结果RLO存入MCR区，MCR区是否为NO取决于RLO，RLO=0时MCR区为NO，在此条件下，区内的逻辑结果受MCR总控，既如A I 4.0 = Q 8.0一句，只要MCR区为NO，Q 8.0即为零，而不管I 4.0的状态如何！