Beetlejuice (NES)/Game Mechanics and Techniques - Revision history

Ktwo: /* Buffered inputs */

2025-08-04T19:54:46Z

‎Buffered inputs

Ktwo: /* Flies/Frogs/Golden beetles/Octopus/Scorpions (side view) */

2025-08-04T19:44:00Z

‎Flies/Frogs/Golden beetles/Octopus/Scorpions (side view)

Ktwo: /* Flies/Frogs/Golden beetles/Octopus/Scorpions (side view) */

2025-08-04T19:42:49Z

‎Flies/Frogs/Golden beetles/Octopus/Scorpions (side view)

Ktwo: /* Cloud elevators */

2025-08-04T19:15:36Z

‎Cloud elevators

Ktwo: /* Cavern monsters */

2025-08-04T19:14:49Z

‎Cavern monsters

Ktwo: /* Cavern monsters */

2025-08-04T19:13:46Z

‎Cavern monsters

Ktwo: /* 3rd boss (Level 3-4) */

2025-08-04T18:57:21Z

‎3rd boss (Level 3-4)

Ktwo: /* 1st boss (Level 1) */

2025-08-04T18:54:02Z

‎1st boss (Level 1)

Ktwo: /* Movement mechanics */

2025-08-04T18:43:07Z

‎Movement mechanics

Ktwo: /* RNG */

2025-07-28T08:42:52Z

‎RNG

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 * The map screen to the afterlife level can be buffered by just keep holding up
 * Hold A (and up/down) while opening the inventory to select a scare
-* Three inputs can be buffered at once when entering a shop. Hold select (+ left/right) + down while to buy an item (, to the left/right of the default position) and exit the shop on the first possible frame.
+* Three inputs can be buffered at once when entering a shop. Hold select (+ left/right) + down to buy an item (, to the left/right of the default position) and exit the shop on the first possible frame.
 * Hold any button during the "skeleton" death animation to re-spawn on the first possible frame (the same does not work falling into a pit). Note that this works in both the side view as well as the overhead levels.
-* Hold up during a fall to buffer the re-spawning on the first possible frame
+* Hold up during an off-screen fall to buffer the re-spawning on the first possible frame
 <br  />

@@ Line 257: / Line 257: @@
 The frogs have an unwanted property that can easily be encountered both during casual play and a speedrun. If the frog jumps "sufficiently" off-screen, it will re-spawn with its position reset and jump again:<br  />
 [[File:Respawning_frog.gif|100px]]<br  />
-This is not a graphical glitch and should be considered in areas where close calls with frogs are part of the route. There is additional risk of getting hit and resetting the frog's position can lead to the frog ending up blocking a series of tight movements.<br  /><br  />
+This is not a graphical glitch and should be considered in areas where close calls with frogs are part of the route. There is additional risk of getting hit and resetting the frog's position can lead to the frog ending up blocking a series of tight movements. Allowing the screen to scroll sufficiently will prevent this from happening.<br  /><br  />
 ''Note: the game only checks for a specific enemy every 12 frames, which introduces some variability to when the individual enemies spawn in a playthrough''

@@ Line 253: / Line 253: @@
 <br  />
-These enemies are activated by the screen having scrolled sufficiently close to them (Beetlejuice's position doesn't matter). They always spawn in the same locations and move predictably, according to their timers.<br  />
+These enemies are activated by the screen having scrolled sufficiently close to them (Beetlejuice's position doesn't matter). They always spawn in the same locations and move predictably, even when off-screen, according to their timers.<br  /><br  />
 ''Note: the game only checks for a specific enemy every 12 frames, which introduces some variability to when the individual enemies spawn in a playthrough''

@@ Line 214: / Line 214: @@
 <br  />
-$463 runs in the background when entering the house (level 1). When a switch is flipped, the corresponding elevator will teleport to wherever in the cycle $463 is and starts to move from there. A cloud cycle is 192 frames.<br  />
+$463 runs in the background when entering the house (level 1). When a switch is flipped, the corresponding elevator will teleport to wherever in the cycle $463 is and start to move from there. A cloud cycle takes 192 frames.<br  />
-−
 ===Door to Saturn===

@@ Line 203: / Line 203: @@
 ''Note4: there is limited room for manipulation. One can for example consider jumping into the fight so Beetlejuice moves towards the vertical edge of the area of potential "virtual positions" that was used to determine the projectile direction.<br  /><br  />
-Some simplified estimations of how likely Beetlejuice is to get hit by a projectile indicated that there is roughly a 1/3 chance when Beetlejuice is far away (and standing still). The risk of getting hit increases as Beetlejuice stands closer to the cavern monster, up to roughly a 50% of getting hit when Beetlejuice stands as close as possible to the cavern monster. If Beetlejuice moves around, the probabilities of getting hit will change. Running towards the cavern monster will increase the risk of getting hit, while jumping will slightly decrease it.<br  /><br  />
+Some simplified estimations of how likely Beetlejuice is to get hit by a projectile indicated that there is roughly a 1/3 chance when Beetlejuice is far away (and standing still). The risk of getting hit increases as Beetlejuice stands closer to the cavern monster, up to roughly a 50% of getting hit when Beetlejuice stands as close as possible. If Beetlejuice moves around, the probabilities of getting hit will change. For example, running towards the cavern monster will increase the risk of getting hit, while jumping will slightly decrease it.<br  /><br  />
 When the monster has been defeated, the game will try to center the screen around Beetlejuice (checks at instructions $911F and $9124, for the y- and x-directions, respectively). Until then, the game doesn't take player inputs, freezing Beetlejuice in place when on the ground. However, the speed is not reset, so if Beetlejuice is in the air when the last attack connects, he'll continue jumping. If the screen catches up and centers around Beetlejuice during this jump, the flag in $5F8 is unset and Beetlejuice will be able to continue without being frozen in place upon landing. The conditions for this to happen are fairly strict though and not trivial to set up even in a TAS, as it requires a correct combination of positioning (including sub-pixels) and speed. The time save is only counted in frames, but still mentioned here for its applications in TASes.

@@ Line 118: / Line 118: @@
 This boss works in essentially the same way as the first one, but with a few different parameters. The ball speed is always 1 pixel/frame (instruction $C95D). When the shot flag is not set, the boss will kick a ball based on a check every 16 frames (based on the global timer in $3BB) with a 50% chance (instruction $C93C).<br  />
-===3rd boss (Level 3-4)===
+===3rd boss (graveyard level)===
 [[File:3rd_boss.PNG|75px]]<br  />
 * $537 - Magic timer

@@ Line 110: / Line 110: @@
 <br  />
-When the shot flag is not set, the RNG function is called every 16 frames, based on the global timer ($3BB). One of the results is compared with 0x46 (instruction $C8F9). If it's less, the boss will shoot a puff of spray. Since the player neither knows the global timer, nor the RNG values, shots can be considered random and outside player control with a probability of (1/16)*(70/256) ~ 1/59 per frame. The fastest method to defeat the boss is to run straight at him and fire off three quick shots. This way, a typical fight takes ~80 frames, which will allow 5 or 6 opportunities for the boss to fire. The success rate will therefore be between (186/256)^6 ~15% and (186/256)^5 ~20%. A slightly more consistent method is to introduce a jump after the first scare attack, regardless of if the boss has actually fired or not. The fight will then typically take ~100 frames. It's not as straight-forward to calculate the success rate in this case and has not been attempted, but it's noticeably more consistent.<br  /><br  />
+When the shot flag is not set, the RNG function is called every 16 frames, based on the global timer ($3BB). One of the results is compared with 0x46 (instruction $C8F9). If it's less, the boss will shoot a puff of spray. Since the player neither knows the global timer, nor the RNG values, shots can be considered random and outside player control with a probability of (1/16)*(70/256) ~ 1/59 per frame. The fastest method to defeat the boss is to run straight at him and fire off three quick shots. This way, a typical fight takes ~80 frames, which will allow 5 or 6 opportunities for the boss to fire. The success rate will therefore be between (186/256)^6 ~15% and (186/256)^5 ~20%. A slightly more consistent method is to jump and shoot, regardless of if the boss has actually fired or not. The fight will then typically take ~100 frames. It's not as straight-forward to calculate the success rate in this case and has not been attempted, but it's noticeably more consistent.<br  /><br  />
 The speed of the puff is either 1 or 2 pixels/frame and with a sub-speed that is a multiple of 0x40 (a sub-speed >0 slows down the puffs). Both the main and sub speeds directly depend on $24 (the first RNG-address) and should therefore be equally distributed with no possibility to predict in real-time.<br  />

@@ Line 13: / Line 13: @@
 The "saved" position is used to determine where Beetlejuice will respawn after a death. It's updated every 8 ticks of the global timer. However, it's only updated if Beetlejuice is running, but not while in the air. This can be used to set up "death warps" by jumping across a level, die and warp back instead of back-tracking normally.<br  /><br  />
-An oddity, albeit a very small one, is that there is an immobility frame if Beetlejuice runs off a platform. Although the screen scrolling will in many cases be more important than an immobility frame, some platforms can be jumped off of without consequence for one frame saved.
+An oddity, albeit a very small one, is that there is an immobility frame if Beetlejuice runs (or drops) off a platform. Although the screen scrolling will in many cases be more important than an immobility frame, some platforms can be jumped off of without consequence for one frame saved.
 <br  />

@@ Line 97: / Line 97: @@
 # Starting at $909A, only two of the RNG-values are updated. This is done based on Beetlejuice's x- and y-position (influencing $24 and $25, respectively).
-The first two algorithms are used throughout the game with what appears to be a room-dependent frequency (usually every 2-8 frames), with additional calls if there are RNG-dependent enemies present. The sub-routine for the third algorithm is only used in the side-scrolling levels and only called every 256 frames. Room 0x09 (the very first overhead room in the game) is special in the sense that the RNG is never updated in this room.<br  /><br  />
+The first two algorithms are used throughout the game with what appears to be a room-dependent frequency (usually every 2-8 frames), with additional calls if there are RNG-dependent enemies present. The sub-routine for the third algorithm is only used in the side-scrolling levels and only called every 256 frames ($3BB = 0x1D). Room 0x09 (the very first overhead room in the game) is special in the sense that the RNG is never updated in this room.<br  /><br  />
 There is no indication that the RNG-values would be possible to manipulate in real-time. In TAS-conditions, the RNG is fairly "stiff" and can only be manipulated to a certain extent (unless introducing waiting frames is considered). In the side-view levels, it's done by changing Beetlejuice's position before '3' is called. In the overhead levels, it's mainly done by throwing a potion at different times or in some cases defeating enemies on different frames. In the room with Delia (0x19), picking up the mouse and scaring off Delia on different frames can also be used to influence the RNG. These methods have in common that they influence the number of calls to the RNG sub-routine.<br  /><br  />