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C++ if() statements implements a comparison and a jump opcode. By way of example, consider the following code:
if( x 100 ) generates a compare and jump opcode (cmp and jne).
The cmp opcode compares the value of x with 64h (100 decimal) which will either set or clear ZF (the zero flag). If x is 100, then ZF will be unset, otherwise it will be set.
The jne (jump if not equal) opcode tests ZF. If it is set, then x was not 100 and the operand (1181934h) is passed to the PC register (the program counter). The IR (instruction register) will fetch the value from PC on the next cycle. This ultimately causes execution to jump to the statement immediately following the else clause of the if statement (the second printf statement).
If ZF is not set, then x really was 100 and the PC register remains unaffected. At that point the PC register will contain the value 0118191Bh, which is the next instruction after the if statement, thus execution simply falls through to the first printf statement. After processing the printfinstructions, execution will reach the else clause which simply forces a jump to bypass the second printfstatement.
Thus, one of the printf statements is executed and then execution continues from the instruction at 118194Bh, which is not shown but is the next instruction after the second printfstatement.
A nested if is an if statement that is nested within another if statement.
int x=1, y=2;
if( x==1 )
{
if( y==1 ) // nested if.
{
// x==1 and y==1
}
else if( y==2 ) // another nested if.
{
// x==1 and y==2
}
else
{
if( y==1 ) // yet another nested if.
{
// x!=1 and y==1
}
else if( y==2 ) // and another nested if.
{
// x!=1 and y==2
}
}
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