TSTNE and the Zero Bit

Let's take the simple lines of code to test non-equality of two items and the results of the conditional branch
 
    TSTNE  A B
    GOTO  NOTSAME
SAME:
    do some stuff...
 
NOTSAME:
     do some other stuff...
 
 
 
Now the TSTNE usually performs the usual mental, behind the scenes,  subtraction of the two quantities like this:
 
     A - B = ?
 
If equal,    then A-B = 0,  and the zero bit should be set, from the result, indicating that, right?
     Right! But it is inverted to what we need and the test is for NOT equality so we have to invert the zero bit..
 
Our zero bit for each case becomes:
    A ==  B , zero bit = 0 (false)
    A =/= B,  zero bit = 1 (true)
 
 
Like all  TSTxx   PLC code lines, the zero bit indicates  "the result of the expression tested is true" and if true  executes the next line:
 
    GOTO NOTSAME
 
resulting in a branch of execution in the PLC code. in this case.
 
 
 
Similarly if the result of doing the NOT (A - B = 0)  mental math is false  (They happen to be equal)  then the ELSE condition happens with the Zero bit 0.  All that means is the PLC code skips the next TRUE line after the TSTNE and executes:
 
SAME:
      do some stuff.....
 
 
 
TSTEQ
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You may have guessed by now that the TSTEQ performs exactly the same mental subtraction of the two operands  except without  the Zero Bit result being  inverted.

Larry,
 
You are correct, zerobit is a variable can be referenced in the PLC code ZBIT, that reflect last value being assigned to any variable or IO was zero, including from IO operations, from bit manipulations, and from comparison or other calculations.   Hoops, it was not documented in user guide, but exist for some times ---we added because user request from this forum.  ZBIT is read only.
 
TSTEQ a b
a == b  ZBIT 1
TSTNE a  b
a != b   ZBIT  1
 
...
SIND 0
ZBIT 0

Is the Zerobit also accessible via cgi script?
thanks

ZBIT is readable in PLC, but can only be set through PLC operations.