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A while ago, I was able to implement a 60 fps overworld option in my rom hack Shin Pokemon. It is based on an older commit of pokered disassembly from 2018, but most things are still in the same place. Anyway, I've been wanting to write down how I did this for others to follow. With any luck this will be picked up by search engine crawlers, and some poor sap years from now won't have to waste hours on Google sifting through dead links and vague hints.
Alright! Let's jump right in.
Go to home/overworld.asm and scroll down to OverworldLoop and OverworldLoopLessDelay. Here's the key. Notice that DelayFrame is called in OverworldLoop then you fall through to OverworldLoopLessDelay and call DelayFrame again. This is what causes the overworld to run at 30 fps. The gameboy is locked to 60 fps, but the overworld is updating every 2 frames.
Comment-out that first delay. Congrats, you now have a 60 FPS overworld. Recompile and fire up the game.
What's that? All the movement and animation speeds for overworld objects are messed up and twice as fast? Well, I didn't expect you to be so picky. Fine then. Let's continue on and do things properly.
Go to wram.asm and scroll down to wSpriteStateData2. This is a data structure used by overworld objects. That's tuff that can move around on a map. The 'x' in each address refers to the number of the object on the loaded map. The player character is always object 'x' = 0, so each map can have 15 other objects (1 through F) on it at a time. The really important thing here is address C2xA. That's an unused part of the structure, so comment that as being used for 60FPS. We're going to snag it for our purposes. Specifically, it is going to toggle between 0 and 1 to indicate if object data needs to update or be skipped. This is because, at 60 fps, many things need to only update every other frame.
The 60FPS setting needs to be able to be turned on and off. All that's needed is 1 bit in an unused byte of wram. I used bit 4 of the wram address titled wUnusedD721 out of convenience. 60FPS mode will be active when this bit = 1. Jot this down in your notes. Create the Check60fps function at the bottom of the home/overworld.asm file to easily check for this, and you can now add this toggle to OverworldLoop like so. We'll be doing stuff like this a lot.
Now let's do one of the easier bits. Time to fix jumping ledges. Go to engine/overworld/player_animations.asm and scroll down to the end. You're going to add a new function called Ledge60fps. This function takes the jumping player's incremented Y-coord index as an input in the A register. Running at double framerate means this should only increment every other frame.
Now scroll up to the function _HandleMidJump and call Ledge60fps just after A gets incremented at the beginning.
Go back to home/overworld.asm because there is more stuff to do. Time to fix basic player movement.
Scroll down to OverworldLoopLessDelay.noCollision and notice the counter value of $08.
Modify this so that 60FPS mode doubles this to $10.
Scroll down to AdvancePlayerSprite.afterUpdateMapCoords and notice the counter value of $07.
Modify this so that 60FPS mode doubles this to $0F.
Scroll down to AdvancePlayerSprite.scrollBackgroundAndSprites and notice how some X and Y delta values are shifted left in order to double them.
Modify this so that 60FPS mode does not double them.
Doing great so far!
Walking animations currently run at double-speed. Let's fix that.
Open up engine/overworld/movement.asm and scroll down to the bottom to create a new function called sprite60fps.
No go to UpdatePlayerSprite.moving where you will call the sprite60fps function at a specific spot.
NPCs update their movement twice as fast, so this needs to be corrected.
Go to UpdateSpriteMovementDelay and modify it as such.
UpdateSpriteInWalkingAnimation also needs to have several things update every other frame. Modify it as such.
Finally the last part. DoScriptedNPCMovement needs updates for the rare instances where the player character does scripted movement following behind a NPC. And example of this is when you follow Oak into his lab at the start of the game.
And finish things off with some minor updates to AdvanceScriptedNPCAnimFrameCounter.
And there you go! Setting bit 4 of wUnusedD721 will put the game into 60FPS mode. I'll leave in-game activation to you.
Alright! Let's jump right in.
Go to home/overworld.asm and scroll down to OverworldLoop and OverworldLoopLessDelay. Here's the key. Notice that DelayFrame is called in OverworldLoop then you fall through to OverworldLoopLessDelay and call DelayFrame again. This is what causes the overworld to run at 30 fps. The gameboy is locked to 60 fps, but the overworld is updating every 2 frames.
Comment-out that first delay. Congrats, you now have a 60 FPS overworld. Recompile and fire up the game.
What's that? All the movement and animation speeds for overworld objects are messed up and twice as fast? Well, I didn't expect you to be so picky. Fine then. Let's continue on and do things properly.
Go to wram.asm and scroll down to wSpriteStateData2. This is a data structure used by overworld objects. That's tuff that can move around on a map. The 'x' in each address refers to the number of the object on the loaded map. The player character is always object 'x' = 0, so each map can have 15 other objects (1 through F) on it at a time. The really important thing here is address C2xA. That's an unused part of the structure, so comment that as being used for 60FPS. We're going to snag it for our purposes. Specifically, it is going to toggle between 0 and 1 to indicate if object data needs to update or be skipped. This is because, at 60 fps, many things need to only update every other frame.
The 60FPS setting needs to be able to be turned on and off. All that's needed is 1 bit in an unused byte of wram. I used bit 4 of the wram address titled wUnusedD721 out of convenience. 60FPS mode will be active when this bit = 1. Jot this down in your notes. Create the Check60fps function at the bottom of the home/overworld.asm file to easily check for this, and you can now add this toggle to OverworldLoop like so. We'll be doing stuff like this a lot.
Now let's do one of the easier bits. Time to fix jumping ledges. Go to engine/overworld/player_animations.asm and scroll down to the end. You're going to add a new function called Ledge60fps. This function takes the jumping player's incremented Y-coord index as an input in the A register. Running at double framerate means this should only increment every other frame.
Now scroll up to the function _HandleMidJump and call Ledge60fps just after A gets incremented at the beginning.
Go back to home/overworld.asm because there is more stuff to do. Time to fix basic player movement.
Scroll down to OverworldLoopLessDelay.noCollision and notice the counter value of $08.
Modify this so that 60FPS mode doubles this to $10.
Scroll down to AdvancePlayerSprite.afterUpdateMapCoords and notice the counter value of $07.
Modify this so that 60FPS mode doubles this to $0F.
Scroll down to AdvancePlayerSprite.scrollBackgroundAndSprites and notice how some X and Y delta values are shifted left in order to double them.
Modify this so that 60FPS mode does not double them.
Doing great so far!
Walking animations currently run at double-speed. Let's fix that.
Open up engine/overworld/movement.asm and scroll down to the bottom to create a new function called sprite60fps.
What this will do is toggle the value in the C2xA address for the x'th object being referenced every frame. If 60FPS mode is inactive, it will keep this value as 0. If it is active, it will be a value of 1 every other frame. This value is also returned in the B register.
No go to UpdatePlayerSprite.moving where you will call the sprite60fps function at a specific spot.
ld a, [hl] followed by inc a is updating the animation counter. You only want to do this every other frame. Calling sprite60fps and subtracting B from A does this for you by undoing the last increment of A every other frame.
NPCs update their movement twice as fast, so this needs to be corrected.
Go to UpdateSpriteMovementDelay and modify it as such.
UpdateSpriteInWalkingAnimation also needs to have several things update every other frame. Modify it as such.
Finally the last part. DoScriptedNPCMovement needs updates for the rare instances where the player character does scripted movement following behind a NPC. And example of this is when you follow Oak into his lab at the start of the game.
And finish things off with some minor updates to AdvanceScriptedNPCAnimFrameCounter.
And there you go! Setting bit 4 of wUnusedD721 will put the game into 60FPS mode. I'll leave in-game activation to you.