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”.

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.

  STA flag

However, sometimes our register values are unknown and must be preserved.

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.

 SEC
 ROL flag

However, sometimes the carry state is unknown and must be preserved.

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.

 INC flag

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:

: INC flag
  BEQ :-

The branch will be taken incredibly rarely.