====== Set a byte to non-zero ======

//by White Flame//

Consider a flag in memory that is initialized to zero, but should become non-zero based on some check.  The byte will later be polled to invoke some behavior and reset to zero.  Sometimes small/fast programs need to get clever with this very simple operation depending on how the registers are constrained.

In this page, the "MNZ" operation means "Make Non-Zero".


==== Using Register Values ====

If a register is known to contain a non-zero value (or is free to immediately set its value), then a basic STA/STX/STY will MNZ.  This is the only way to perform this operation without affecting the N or Z processor flags.
<code autoit>
  STA flag
</code>
However, sometimes our register values are unknown and must be preserved.


==== Rolling a Carry Bit ====

ROL/ROR'ing in a set (or known set) carry bit will guarantee a byte becomes non-zero without affecting the registers.  This does destroy the carry bit, unless it is guaranteed that MNZ will not be performed on a flag more than 7 times before checking & resetting to zero.
<code autoit>
 SEC
 ROL flag
</code>
However, sometimes the carry state is unknown and must be preserved.


==== Using INC/DEC ====

Using INC or DEC to pull a value away from zero allows a flag to be MNZ'd up to 255 times, while preserving all of A, X, Y, and the Carry flag.

<code autoit>
 INC flag
</code>

However, sometimes we do not know how often the MNZ operation will be done between resets.

This final means ALWAYS guarantees a flag byte becomes non-zero without affecting A, X, Y, or C, no matter how often it's run between resets:
<code autoit>
: INC flag
  BEQ :-
</code>
The branch will be taken incredibly rarely.