The carry added well the add operation automatically adds the carry

166 Programming the 65816

The comparison operation—is VALUEl equal to VALUE2, for exam­ple—is implemented on the 65x, as on most processors, as an implied subtraction. In order to compare VALUEl to VALUE2, one of the val­ues is subtracted from the other. Clearly, if the result is zero, then the numbers are equal.

This kind of comparison can be made using the instructions you already know, as Fragment 9.5 illustrates. In this fragment, you can see that the branch to TRUE will be taken, and the INC VAL instruction never executed, because $1234 minus $1234 equals zero. Since the results of subtractions condition the z flag, the BEQ instruction (which literally means "branch if result equal to zero"), in this case, means "branch if the compared values are equal."

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