Screen transitions (sometimes referred to as “doors”) in Metroid II use a bytecode interpreter to perform the various actions needed when moving between different rooms, such as changing the tileset, activating a fadeout, and so forth. A basic knowledge of the screen transition bytecode language and how to easily edit it are essential skills for making more elaborate hacks of this game.
It is recommended that you have a basic understanding of Level Data Banks before continuing on.
Each screen on each map has a screen transition index associated with it. As long as the index is not zero, the screen transition corresponding to the room will be activated once Samus touches any edge of the screen. (The pointer table indexed by the transition number starts at 0x142E5 in rom.)
Once the game identifies the script to use, it copies the next $40 bytes to
$D700 in RAM (which is the easiest place to inspect how they work). From there, it interprets the script byte-by-byte in a big loop. (Note that this means the maximum effective size of a door script is 64 (0x40) bytes, but you will probably never hit that limit.)
The interpreter first checks if it has reached the end of the script ($FF), and exits if that is the case. After that, it masks out the lower nybble of the current byte (e.g. $B2 becomes $B0), and then proceeds through a bunch of if-else statements to find the routine that matches the upper nybble (the “B” in $B2, for example), and then executes the command. After executing the command, the interpreter loops back to the beginning and repeats the process for the next operation.
(Note that all 16-bit values (pointers, etc.) are assumed to be little-endian, as usual.)
Here is a list of valid tokens/operators used by the bytecode interpreter
Copies a block of data starting at a specified bank and address to somewhere in RAM.
01 bb ssss dddd llll- Saves bb:ssss as the source of a BG tileset (meaning the tiles will be loaded when reloading a save)
02 bb ssss dddd llll- Saves ssss as the source of a sprite tileset (meaning the tiles will be loaded when reloading a save)
0* bb ssss dddd llll- Just copies the data.
This doesn't seem to be used that often, probably because it's so verbose. The B* operator is typically used instead for loading sprite and background graphics.
Copies the desired metatile table to $DA00-$DBFF in WRAM.
0 - finalLab 1 - ruinsInside 2 - plantBubbles 3 - queen 4 - caveFirst 5 - surface 6 - lavaCavesEmpty 7 - lavaCavesFull 8 - lavaCavesMid 9 - ruinsExt
The lava caves adjust their lava levels by changing their metatiles to sets with more or fewer lava tiles during door transitions.
The metatile table pointer list begins at
0x23F1A in ROM. (For some reason, these pointers slightly out of order compared to how the data is stored in the rom.)
Copies the desired collision table to $DC00-$DCFF in WRAM.
Collision Tables 0 - plantBubbles 1 - ruinsInside 2 - queen 3 - caveFirst 4 - surface 5 - lavaCaves 6 - ruinsExt 7 - finalLab
The collision table pointer list begins at 0x23EEA in ROM.
Copies the desired solidity indexes to WRAM, for Samus, enemies, and Samus' projectiles.
Basically, the way it works is that if an 8×8 tile is less than the specified index, then it is treated as a solid. While Samus, enemies, and projectiles may use different values, the process remains the same.
Solidity Values 0 - plantBubbles 1 - ruinsInside 2 - queen 3 - caveFirst 4 - surface 5 - lavaCaves 6 - ruinsExt 7 - finalLab
Note that (by default) these match up perfectly with the collision values previously.
The solidity indexes are stored in a table starting at 0x23EFA. Each entry row is four bytes wide, though only three are used.
The bytes are copied to these locations:
Warps Samus to the specified map and screen.
Note: Place the x/y coordinate one screen “behind” the desired destination.
This opcode is used only once, when Samus retreats from the last boss fight by falling through the hole in the floor. It performs several tasks:
6* xx yy
This is used towards the end of the game to increase the acid damage from $02 to $06.
For reference, the default starting values are $02 and $08. (Note that damage values for Samus in this game are binary-coded decimal.)
This opcode is used only once, when leaving the last boss room after defeating her. It performs the following:
Handles a lot of special stuff regarding the queen fight, but not everything.
8a bbbb cccc dddd eeee
9* nn xxxx
“If the amount of Metroids remaining is less than or equal 'nn', then jump to and do transition 'xxxx' instead.”
This is how the game handles lowering (and raising) acid levels. Keep in mind that earthquakes are handled elsewhere.
Note that this uses the real Metroid counter (memory address $D089), which also accounts for the Metroids in the endgame. Also keep in mind that this value is in binary coded decimal.
When changing tileset, this is typically the first operator in the script.
B1 bb xxxx- Load BG graphics page (128 tiles) to $9000 in VRAM.
B2 bb xxxx- Load Sprite graphics page (64 tiles) to $8B00 in VRAM.
Note that these opcodes update the variables that keep track of what tilesets to use when loading a save.
BG Graphics Pages 07:4000 plantBubbles 07:4800 ruinsInside 07:5000 queenBG 07:5800 caveFirst 07:6000 surfaceBG 07:6800 lavaCavesA 07:6D30 lavaCavesB 07:7260 lavaCavesC 08:71BC finalLab 08:79BC queenSPR
Sprite Graphics Pages 06:5920 enemiesA 06:5D20 enemiesB 06:6120 enemiesC 06:6520 enemiesD 06:6920 enemiesE 06:6D20 enemiesF 06:7120 arachnus 06:7520 surfaceSPR 08:59BC metAlpha 08:5DBC metGamma 08:61BC metZeta 08:65BC metOmega 08:69BC ruinsExt 08:71BC finalLab 08:79BC queenSPR*
* Note: queenSPR is loaded using the “0*” operator because it uses a nonstandard amount of tiles.
0 - No change in music. - Silences the roar from "song" A below 1 - "The Last Metroid" 2 - Queen Fight 3 - Inside Ruins 4 - Main Tunnels 5 - Ambience 1 6 - Ambience 2 7 - Ambience 3 (bugs) 8 - Omega Metroid Area 9 - Final Ruins A - No music: SFX - Metroid Queen roar (persists) B - Final Alarm C - Metroid Fight D - Ambience 4 E - SFX: Earthquake F - Metroid Defeated
This loads the graphics for the orb, the desired item, the item font (well, up to the number 2), and the corresponding text string to VRAM.
0 - Save ... (just for the message) 1 - Plasma Beam 2 - Ice Beam 3 - Wave Beam 4 - Spazer 5 - Bombs 6 - Screw Attack 7 - Varia Suit 8 - High Jump Boots 9 - Space Jump A - Spider Ball B - Spring Ball C - Energy Tank (unused) D - Missile Tank (unused) E - Energy (i.e. refill; again, unused) F - Missiles (i.e. refill; unused)
Note that, for the last 4 items, their sprites are always in VRAM, and they don't cause text strings to appear, so this is unnecessary for them.
The game does not account for operators in the form of E* or F*. Using these will cause the game to freeze, because the interpreter will get stuck in an infinite loop. However, an intrepid hacker could use these to extend this language to do other radical things.
Each screen can only have one transition assigned to it, meaning that if (for example) you need the left and right exits of a room to lead to different areas, then the room needs to be at least 2 screens wide (this limitation becomes very apparent when examining the original game's map design).
Also, the only conditional operator that exists is for the metroid count. Checking any other byte of the save data (such as if a particular enemy or item has been collected) is unsupported.
With the disassembly, however, both of these limitations could be overcome relatively trivially.
Transition scripts can be edited in one of two ways:
1) Using the transition editor in LAMP.
For reference, the macros the disassembly uses are as follows:
|00 bb ssss dddd llll||COPY_DATA|
|01 bb ssss dddd llll||COPY_BG|
|02 bb ssss dddd llll||COPY_SPR|
|60 xx yy||DAMAGE|
|8a bbbb cccc dddd eeee||ENTER_QUEEN|
|90 nn xxxx||IF_MET_LESS|
|B1 bb xxxx||LOAD_BG|
|B1 bb xxxx||LOAD_SPR|
This the transition from the save room in the first ruins to the exterior area:
A0 B1 08 BC 69 26 36 19 B2 06 20 5D C5 49 E7 FF
Editing this by hand would by tedious and error-prone. However, with the disassembly it becomes fairly rudimentary:
door10C: FADEOUT LOAD_BG gfx_ruinsExt COLLISION $6 SOLIDITY $6 TILETABLE $9 LOAD_SPR gfx_enemiesB SONG $5 WARP $9, $E7 END_DOOR
Notice how this is largely human-readable, and how one can use the label names for the graphics locations in the ROM, rather than hardcoded offsets.