How Zelda II Overworld Monster Spawns Work

How Zelda II Overworld Monster Spawns Work
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(Reworded quite a bit from the original, taken from the submission text for Inzult & Rising Tempest's "No Zipping or Unintended Exits" TAS found at http://tasvideos.org/4266S.html to make a bit more layman's sense, as the original is worded in a way that isn't as easy to understand.)

There are 2 types of overworld monster spawns that can happen; one of these is based on the steps you've taken, the other is based on time, and in most areas, both of these run concurrently.

Time-Based
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The time-based monster spawn timer is always set to 8 "ticks" upon exiting a side-scrolling area; all ticks aside from the first are 21 frames long (so generally, 1 Tick = 21 Frames); the first tick is variable depending where you are on the global timer when exiting a side-scrolling area.  (How to discern how long the first tick will be isn't mentioned in the original text.)

This counts down from however many ticks are on it, and when it reaches 0, if you're standing on a tile that can spawn monsters, they'll spawn.  If you're traveling on a path, it simply stays at 0 until you step off the path onto terrain that *can* spawn monsters (at which point, of course, they'll immediately spawn).

So when you first exit a side-scrolling area, a spawn will occur in about 168 frames (2.8 seconds or so), give or take a bit, depending where you were on the global timer; this approximate timeframe is pretty familiar to speedrunners since it tends to be pretty consistent.

If you're still standing on the overworld when the tick counter reaches 0 and enemies spawn, it will reset to a new number of ticks; the number of ticks is based upon what kind of terrain you're standing on when a time-based spawn happens, thus meaning some terrain types are safer than others:
* Grass or Swamp: 32 Ticks (672 Frames; 11.2 seconds)
* Desert or Forest: 24 Ticks (504 Frames; 8.4 seconds)
* Graveyard: 9 Ticks (189 Frames; 3.15 seconds)
* Lava: 3 Ticks (63 Frames; 1.05 seconds)

Step-Based
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The step counter for random encounters counts up from 1 to 255, increasing by 16 with every step.  When it rolls over from 255 back to 1, monsters spawn.  The terrain you're standing on does not affect this rule in any way.  If you're standing on a path when the 16th step is taken, there simply won't *be* a step-based monster spawn for that step cycle, and the step counter just resets without anything happening.

In northwest Hyrule, in the area around the North Palace, and around Palace 1, the step counter isn't active; as such, you cannot get step-based monster spawns in those regions (though the time-based counter still applies).