Hello friends,

last night while streaming I came across a very exciting glitch, health underflow. As many of you know, health is an integral part of the run and taking the time to heal costs precious seconds.

Well, last night I had extremely low HP, apparently zero, got hit by an enemy without dying and while in a flashing state performed a screen transition and poof! Health bar was gone. I checked by damaging myself more that this was indeed an underflow as after several hits it would return to the top.

My question now is how to replicate it. I was able to reproduce the outcome several times as demonstrated in the video attached but couldn't get it on all enemies, and by the end of the night I couldn't seem to get it again.

Here is the Twitch video:

Here is the amarec recording:

I have uploaded an example on emulator in the resource section: https://www.speedrun.com/alttpnes/resources this will work on patched and unpatched ROMs, which will not be provided here but are not hard to find. Emulator used was FCEUX.

It appears there are several variables that affect this glitch but I have found a setup that consistently works. First go into a dungeon, lose some HP, pick up a single heart, get your HP down to zero visible then leave the dungeon, get hit by an enemy, while in the flashing state move across a screen transition and you should have the glitch.

Picking up hearts in dungeons afterwards does not break the glitch, picking up hearts in the overworld does.

Inside dungeons, when getting hit:

• In blinking state, skip (exit)
• Set the blinking state ($640E=08) and reset its frame counter ($664D=00), which ends the blinking state after 80 frames
• If HP < 2 ($60FD), game over (exit)
• Lose 2 HP

Outside dungeons:

• The blinking state isn't checked, so you can get hit really fast
• Set the blinking state ($60FB=FF) and don't reset the blink counter ($60FC), which ends after 160 frames (40 blinks) total
• Lose 1 HP
• If HP = 0, game over

Inside dungeons, when getting a heart drop:

• If HP >= 48, skip
• Gain 1 HP

Outside dungeons:

• Gain 2 HP
• If HP >= 48, reset to 48

So the way to do it would be:

• Enter a dungeon and get either full life or any even amount (if a heart drop gains a pixel on the life bar, you got it)
• Take damage until HP = 0 (empty bar) then exit
• Getting hit outside will underflow to 255

The underflow happens as soon as you get hit, but doesn't refresh the life bar until you scroll (or open the menu).

This is some really good analysis here Acmlm.

Wow, so you actually get the underflow straight away and the screen transition means nothing.

So if I've read this all correctly, the setup is actually far simpler than I had thought, as long as a heart pickup gets you a pixel on the bar (in a dungeon), then if you get your HP down to 0, the underflow will work. That's way easier to do in runs! No need to keep track of various counts and far less concerns for pickup RNG.