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Compound condition with OR

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• Program control of the if-statement with an compound condition "A or theRest":

  if ( A or theRest ) ----> | statement1; V else +--------------+ TRUE statement2; | eval A |--------+ +--------------+ | | | | FALSE | V | +--------------+ FALSE | | eval theRest |--------|---+ +--------------+ | | | TRUE | | | | | V | | statement1 <---------+ | | | +---------+ | | | | +--------------------+ | | | V | statement2 | | | | +-------->+ | | V

  • If A is true, then A or "something" is also true, regardless what the "something" evaluates to.

  • Therefore, we can branch (jump) to the then-part as soon as we find that A is true.

• Assembler construct that realizes the control flow of the if-statement with an compound condition "A or theRest" is as follows:

  Evaluate "A" (CMP) TRUE Branch on TRUE (!!!) outcome of "condition" to here (A:) ---+ | | | FALSE | V | Evaluate "theRest" (another CMP) | | FALSE Branch on FALSE (!!!) outcome of "theRest" to here (B:) ----|-----+ | | | | TRUE | | V | | A: "statement1" assembler code <--------------------------+ | | | Branch always to there (C:) -------------+ | | | | | B: "statement2" assembler code <---------|----------------------+ | | V | C: +<-------------------------------+ | V

• Example:

     int x, y, a; 
  
     if (x <= a || x >= b)
        x = x + 1;
     else
        x = x - 1;
  

  The flow chart of the above program is:

  Assembler program for this compound if-statement: click here