Siemens SIMATIC S7 System Manual page 175

As shown in the following figure, the CPU uses a logic stack to resolve the control logic. In
these examples, "iv0" to "iv31" identify the initial values of the logic stack, "nv" identifies a
new value provided by the instruction, and "S0" identifies the calculated value that is stored
in the logic stack.
The value is unknown (it could be either a 0 or a 1).
1
After the execution of a Logic push or a Load stack instruction, value iv31 is lost.
2
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Program instructions
7.1 Bit logic
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