Metroid Construction

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ros:locations:ram

======Return of Samus: RAM Map======
An in-progress RAM map to aid in custom assembly and ROM disassembly.

=== Overview ===
<code>
0000-3FFF: 16KB ROM Bank (fixed - bank 0)
4000-7FFF: 16KB ROM Bank (swappable)
8000-9FFF: 8KB Video RAM
A000-BFFF: 8KB Save RAM
C000-DFFF: 8KB Work RAM
E000-FDFF: Echo RAM (unused)
FE00-FE9F: OAM
FEA0-FEFF: Not usable
FF00-FF7F: I/O Ports
FF80-FFFE: High RAM
FFFF:      Interrupt enable register
</code>

=== VRAM ===
<code>
8000-97FF: VRAM tiles
    8000-87AF: Samus
        8080-9F: Arm cannon
    8B00-8EFF: Enemies
        0x8C00-8E2F: Characters if room has message box (probably loaded with chozo?)
    9000-97FF: Background
9800-9FFF: VRAM tilemap
    9800-9BFF: Background
    9C00-9FFF: Window
        9C00: Status bar
        9C20: Message
</code>

=== SRAM ===
<code>
A000-A03F: File 1
    + 00-07: 0123456789ABCDEF
    + 08: Samus' Y position
    + 0A: Samus' X position
    + 0C: Screen Y position
    + 0E: Screen X position
    + 10: Enemy tiles source address (2 bytes)
    + 12: Background tiles source bank
    + 13: Background tiles source address (2 bytes)
    + 15: Metatile definitions source address (bank 8, 2 bytes)
    + 17: Source address of data for $DC00..FF (bank 8, 2 bytes)
    + 19: Bank for current room
    + 1A: Samus passable block threshold
    + 1B: Enemy passable block threshold
    + 1C: Projectile passable block threshold
    + 1D: Samus' equipment
    + 1E: Samus' beam
    + 1F: Samus' energy tanks
    + 20: Samus' health
    + 22: Samus' max missiles
    + 24: Samus' missiles
    + 26: Direction Samus is facing
    + 27: Acid damage
    + 28: Spike damage
    + 29: Real number of Metroids remaining
    + 2A: Song for room
    + 2B: In-game timer, minutes
    + 2C: In-game timer, hours
    + 2D: Number of Metroids remaining
A040-A07F: File 2
A080-A0BF: File 3
A0C0:       Last used save slot

A800-ACE2: Written to in credits [$6:7920..7E02]

B000-B1BF: File 1 Item/Metroid data (0x40 bytes per map)

B200-B3BF: File 2 Item/Metroid data (verify)

B400-B5BF: File 3 Item/Metroid data
</code>

=== WRAM ===
<code>
C000-9F: OAM buffer (gets copied by the sprite DMA)
    + 0: Y position
    + 1: X position
    + 2: Tile number
    + 3: Attributes
         10: Palette
         20: X flip
         40: Y flip
         80: Priority (set: behind background)

C203: Tile Y (see $22BC)
C204: Tile X (see $22BC)
C205: Scroll Y
C206: Scroll X

C215: Tilemap address of ([$C204], [$C203]) (see $22BC)

C308-37: Ch slots of 4 bytes according to $3:6E36
    + 3: XOR'd with 10h in $6E36 every fourth frame

C3A1: Scroll X for LCD interrupt handler type = 1

C3A8: X position of Metroid Queen's head on screen
C3A9: Y position of Metroid Queen's head on screen
C3AA: A two-byte pointer to LCD interrupt data

C3D2: Background palette for LCD interrupt handler type = 1
C3D3: Metroid Queen's health

C3EF: Set to 1 in $3:6E36 if 0 < [Metroid Queen's health] < 32h, probably an aggression flag

C3F1: Set to 1 in $3:6E36 if 0 < [Metroid Queen's health] < 64h, probably an aggression flag

C407: Set to [$D069] in $2:4000

C40F: bottom-most visible metatile row index in map * $10 (big-endian word).
C411: top-most visible metatile row index in map * $10 (big-endian word).
C413: right-most visible metatile column index in map * $10 (big-endian word).
C415: left-most visible metatile column index in map * $10 (big-endian word).

C417: Number of tile that Baby Metroid is touching.
C418: Set to [$D058] in $2:4000

C422: Cleared by $2EE3, set to 1 by $1FF5
C423: Value for $D00F in $2EE3, cleared by $1FF5
C424: Read by $2EE3, set to [$D078] by $1FF5

C430: Used as index for pointer table at $1:5AB1

C432: Scroll Y two frames ago (according to $3:4000)
C433: Scroll Y one frame ago (according to $3:4000)
C434: Scroll X two frames ago (according to $3:4000)
C435: Scroll X one frame ago (according to $3:4000)

C44B: Checked and set in $2:4000, previous value for $C458 in $239C

C459: Used by $1:7A6C (when saving to SRAM), decides where in C540.. to process for $C900

C45C: Used as index for table at $1:729C, value for $FFEA

C500-FE: Object spawn states (map-specific)
    C500-C53F: Non-persistent object (e.g. enemy) spawn states
        01: Spawned in object table ("visible")
        02: Dead/acquired
        FF: Not spawned
    C540-7F: Persistent object (e.g. item, Metroid) spawn states

C600..C7FF: Enemy data. 20h byte slots according to $30BB, current address is kept in $D05E
    + 0: Flags. If bits 0-3 clear, $30BB executes $30EA on slot. If bit 7 is set sprite is invisible (blinking).
    + 1: Y coordinate. Value for $FFB7 in $30EA
    + 2: X coordinate. Value for $FFB8 in $30EA
    + 3: Metasprite number. Value for $FFB9 in $30EA
    + 4: Priority bit ($80) for sprites. Value for $FFBF in $30EA
    + 5: Sprite flip flags ($20 for horizontal, $40 for vertical)
    + 6: Low nibble is blink frames remaining. Bit 4 is sprite palette flag.
    + A: Frozen frame counter. Increments by 2 every 4 frames. 0 is not frozen, 1-$C4 is frozen, $C5-$D0 is thawing.
    + B: Health
    + 1D: Map object slot index

C900-CABF: Item/Metroid data bytes (0x40 per map, gets saved to SRAM)

CEC0-CFFF: Audio data
    CEC0: Tone/sweep channel sound effect
        CEC0: Tone/sweep channel sound effect to play
                0: Nothing
                1: Jumping
                2: Hi-jumping
                3: Screw attacking
                4: Uncrouching / turning around / landing
                5: Crouching / unmorphing
                6: Morphing
                7: Shooting beam
                8: Shooting missile
                9: Shooting ice beam
                Ah: Shooting plasma beam
                Bh: Shooting spazer beam
                Ch: Picked up missile drop
                Dh: Spider ball
                Eh: Picked up energy drop
                Fh: Shot missile door with beam
                10h: 
                11h: 
                12h: 
                13h: Bomb laid
                14h: 
                15h: Option select / missile select
                16h: Shooting wave beam
                17h: 
                18h: Samus' health changed
                19h: No missile dud shot
                1Ah: 
                1Bh: Metroid cry
                1Ch: Saved
                1Dh: 
                1Eh: Unpaused
        CEC1: Tone/sweep channel sound effect playing

        CEC3: Tone/sweep channel sound effect timer
    
    CEC7: Tone channel sound effect
        CEC7: Tone channel sound effect to play
                0: Nothing
                1: Nothing
                2: Nothing
                3: Metroid Queen cry
                4: Baby Metroid hatched / clearing blocks
                5: Baby Metroid cry
                6: Metroid Queen hurt cry
                7: 
        CEC8: Tone channel sound effect playing
        
        CECA: Tone channel sound effect timer

    CECE-CED4: Would be the wave channel sound effect, but is unused (only cleared) and CEE6 is exclusively used instead.

    CED5: Noise channel sound effect
        CED5: Noise channel sound effect to play
                0: Nothing
                1: Enemy shot
                2: Enemy killed
                3: 
                4: Shot block destroyed
                5: Metroid hurt
                6: Samus hurt
                7: Acid damage
                8: Shot missile door with missile
                9: Metroid Queen cry
                Ah: Metroid Queen hurt cry
                Bh: Samus killed
                Ch: Bomb detonated
                Dh: Metroid killed
                Eh: 
                Fh: 
                10h Footsteps
                11h: 
                12h: 
                13h: 
                14h: 
                15h: 
                16h: Baby Metroid hatched / clearing blocks
                17h: Baby Metroid cry
                18h: 
                19h: 
                1Ah: 
        CED6: Noise channel sound effect playing
        
        CED8: Noise channel sound effect timer

    CEDC: Song to play
            0: Nothing
            1: Baby Metroid
            2: Metroid Queen battle
            3: Chozo ruins
            4: Main caves
            5: Sub caves 1
            6: Sub caves 2
            7: Sub caves 3
            8: Final caves
            9: Metroid hive
            Ah: Item-get
            Bh: Metroid Queen hallway
            Ch: Metroid battle
            Dh: Sub caves 4
            Eh: Earthquake
            Fh: Killed Metroid
            10h: Nothing
            11h: Title
            12h: Samus fanfare
            13h: Reach the gunship
            14h: Chozo ruins, same as 3
            15h: Main caves, no intro
            16h: Sub caves 1, no intro
            17h: Sub caves 2, no intro
            18h: Sub caves 3, no intro
            19h: Final caves, same as 8
            1Ah: Metroid hive, same as 9
            1Bh: Item-get, same as Ah
            1Ch: Metroid Queen hallway, same as Bh
            1Dh: Metroid battle, same as Ch
            1Eh: Sub caves 4, no intro
            1Fh: Metroid hive with intro
            20h: Missile pickup
    CEDD: Song playing
    CEDE: Isolated sound effect to play
            0: Go to $42EA
            1: Go to $432B, play item-get music
            3: Go to $4390 (restores some audio state)
            5: Go to $4335, play missile pickup music
            8: Go to $43FB
            Eh: Go to $433F, play earthquake music
            FFh: $CEDE = 0, $CEDF = 0, go to $42FA
            Otherwise: Go to $42FA
    CEDF: Isolated sound effect playing
            If [$CEDE] = 0:
                2: Go to $43C4 (set by isolated sound effect to play = 3, restores some audio state)
                8: Go to $4418
                Otherwise: Go to $42FA

    CEE4: Current tone/sweep channel sound effect
    CEE5: Current tone channel sound effect
    CEE6: Low health beep / wave channel sound effect
            0: Samus' health >= 50
            1: Samus' health < 20
            2: Samus' health < 30
            3: Samus' health < 40
            4: Samus' health < 50
    CEE7: Current noise channel sound effect

    CF00-60: 
        CF00: Music note offset. Set to [$5F30 + ([song to play] - 1) * 2] & ~1 in $48A0
        CF01: Pointer to value for $CF34/$CF36. Set to [$5F30 + ([song to play] - 1) * 2 + 1] in $48A0
        CF03: Working sound channel (1/2/3/4)
        CF04: Set to 1 if [$CF38] != 0 in $48A0
        CF05: Set to 2 if [$CF41] != 0 in $48A0
        CF06: Set to 3 if [$CF4A] != 0 in $48A0
        CF07: Set to 4 if [$CF53] != 0 in $48A0
        CF08: Checked to mirror $CF0B/0C to $CF10/11 in $497A
        CF09: Pointer to wave pattern data, 10h bytes
        CF0B: Working sound channel sweep / enable
        CF0C: Working sound channel sound length / wave pattern duty
        CF0D: Working sound channel envelope / volume
        CF0E: Working sound channel frequency / noise channel polynomial counter
        CF0F: Working noise channel counter control
        CF10-23: Audio channel options (which conveniently correspond with FF10-23)
            CF10: Tone/sweep channel sweep
            CF11: Tone/sweep channel sound length / wave pattern duty
            CF12: Tone/sweep channel envelope
            CF13: Tone/sweep channel frequency
            CF15: Unused
            CF16: Tone channel sound length / wave pattern duty
            CF17: Tone channel envelope
            CF18: Tone channel frequency
            CF1A: Wave channel enable
            CF1B: Wave channel sound length
            CF1C: Wave channel volume
            CF1D: Wave channel frequency
            CF1F: Unused
            CF20: Noise channel sound length
            CF21: Noise channel envelope
            CF22: Noise channel polynomial counter
            CF23: Noise channel counter control
        
        CF26: Song tone/sweep channel instruction pointer
        CF28: Song tone channel instruction pointer
        CF2A: Song wave channel instruction pointer
        CF2C: Song noise channel instruction pointer
        CF2E: A 12 frame timer
        CF2F-37: Working song sound channel options
            CF2F: Instruction pointer
                F1 ee ss ll: For non-wave channels. Working sound channel envelope = ee, working sound channel sweep = ss, working sound channel sound length / wave pattern duty = ll
                F1 pppp vv:  For the wave channel. Pointer to wave pattern data = pppp, working sound channel volume = vv
                F2 pppp:     $CF01 = pppp
                F3 oo:       Add oo to any played music notes (sets $CF00)
                F4 nn:       Repeat from after this instruction nn times |: (sets $CF31/$CF33)
                F5:          Repeat :| (decrements $CF31)
            CF31: Repeat count
            CF33: Repeat point
            
            CF35: Sound envelope / volume

            CF37: Sound length (according to song instruction F1h)
        CF38-40: Song tone/sweep channel options
            CF38: Set to [$5F30 + ([song to play] - 1) * 2 + 3] in $48A0
            
            CF3F: Set to 1 in $48A0
        CF41-49: Song tone channel options
            CF41: Set to [$5F30 + ([song to play] - 1) * 2 + 5] in $48A0
            
            CF48: Set to 1 in $48A0
        CF4A-52: Song wave channel options
            CF4A: Set to [$5F30 + ([song to play] - 1) * 2 + 7] in $48A0
            
            CF51: Set to 1 in $48A0
        CF53-5B: Song noise channel options
            CF53: Set to [$5F30 + ([song to play] - 1) * 2 + 9] in $48A0
            
            CF5A: Set to 1 in $48A0
        CF5C: Set to D0h by [$CEDE] == 8 (timer), sets $CF3E = $CF47 = $CF59 = $CF5D = $CF5E = [a]
            0: Song play = isolated sound effect to play = 0, disable sound channels
            10h: a = 13h
            30h: a = 25h
            70h: a = 45h, $CF07 = 0, $CF50 = $CF5F = 60h
            A0h: a = 65h
        CF5D: Set to [$CF3E] by [$CEDE] == 8
        CF5E: Set to [$CF47] by [$CEDE] == 8
        CF5F: Set to [$CF50] by [$CEDE] == 8
        CF60: Tone channel frequency tweak. Set to 1 if [$5F30 + ([song to play] - 1) * 2] & 1 in $48A0
    CF61-C1: Mirror of $CF00-60?

    CFC5: Backup of song playing during isolated sound effect

    CFC7: 
            1: Go to $4801
            2: Go to $4846. Set when unpausing: plays unpaused sound effect
            Otherwise: $4819
    CFC8: Flag to go to $4852, set to 40h by $4801
    CFC9: Mirror of $CF10? (tone/sweep channel sweep)

    CFE3: Mirror of $CF09?
    CFE5: Low health beep / wave channel sound effect to play
    CFE6: Low health beep / wave channel sound effect playing
    CFE7: Mirror of $CFE6?

    CFEC: Mirror of $CFED?
    CFED: Audio channel output stereo flags
    CFEE: Quickly guessing wave pattern timer

D008: Metatile top-left
D009: Metatile top-right
D00A: Metatile bottom-left
D00B: Metatile bottom-right
D00C: Samus' previous Y position
D00E: Door transition direction
        1: Right
        2: Left
        4: Up
        8: Down
D00F: Direction facing while spin-jumping or morph-jumping.
        00: Up
        01: Right
        FF: Left
        
D010: Counter for spin-jumping
D011: Nothing. Only cleared
D012: Value for $D060 in $31F1

D020: Samus' pose
        00: Standing
        01: Jumping
        02: Spin-jumping
        03: Running (set to 83h when turning)
        04: Crouching
        05: Morphball
        06: Morphball jumping
        07: Falling
        08: Morphball falling
        09: Starting to jump
        0A: Starting to spin-jump
        0B: Spider ball rolling
        0C: Spider ball falling
        0D: Spider ball jumping
        0E: Spider ball
        0F: Knockback
        10: Morphball knockback
        11: Standing bombed
        12: Morphball bombed
        13: Facing screen
        18: Being eaten by Metroid Queen
        19: In Metroid Queen's mouth
        1A: Being swallowed by Metroid Queen
        1B: In Metroid Queen's stomach
        1C: Escaping Metroid Queen
        1D: Escaped Metroid Queen

D022: += 3 in $08FE during door transition
D023: Direction of screen movement
        10: Right
        20: Left
        40: Up
        80: Down

D027: Samus X position
D029: Samus Y position
D02B: Direction Samus is facing, mirrored from $D81E at save load.
        0: Left
        1: Right
D02C: Samus turning animation counter

D035: Screen right velocity
D036: Screen left velocity
D037: Screen up velocity
D038: Screen down velocity
D039: Set to 0 by load title screen

D03B: Samus' Y position on screen
D03C: Samus' X position on screen, checked for <= A0h in $08FE
D03D: Spider ball direction
        0: In air
        1: On bottom-left corner of ledge
        2: On top-left corner of ledge
        3: On left-facing wall
        4: On bottom-right corner of ledge
        5: On ceiling
        6: Unused
        7: On meet of left-facing wall and ceiling
        8: On top-right corner of ledge
        9: Unused
        Ah: On floor
        Bh: On meet of left-facing wall and floor
        Ch: On right-facing wall
        Dh: On meet of right-facing wall and ceiling
        Eh: On meet of right-facing wall and floor

D045: Samus' equipment, mirrored from $D815 at save load.
        01: Bombs
        02: Hi-jump
        04: Screw attack
        08: Space jump
        10: Spring ball
        20: Spider ball
        40: Varia suit
D046: Debug screen selector index
D047: Tiles update flag (see $FFB1..B6, $2BA3, $27BA)

D04D: Weapon equipped.
        0: Normal
        1: Ice
        2: Wave
        3: Spazer
        4: Plasma
        8: Missile
D04E: Bank

D050: Samus' max energy tanks, mirrored from $D817 at save load.
D051: Samus' health, mirrored from $D818 at save load.
D052: Samus' filled energy tanks, mirrored from $D819 at save load.
D053: Samus' missiles(upper nybble tens, lower nybble ones), mirrored from $D81C at save load.
D054: Samus' missiles(upper nybble thousands, lower nybble hundreds), mirrored from $D81D at save load.
D055: Samus' beam. Saved to SRAM, mirrored from $D816 at save load.
        0: Normal
        1: Ice
        2: Wave
        3: Spazer
        4: Plasma
D056: Samus passable block threshold
D057: Room sprite priority
        0: Sprites over BG
        1: BG over sprites
D058: Bank for current room
D059: Death sequence timer. Used as index into offset table for dest address in $2FE1

D064: Used in $239C as new OAM stack pointer, set to OAM stack pointer in $04DF
D065: VRAM DMA bank (see $FFB1-B6, $2BA3)
D066: Used in v-blank handler. Timer for fade in. Set to 40h by $0CA3, set to 2Fh by $239C when type = Ah, set to A0h by game mode Ah, set to FFh by reached the gunship
D067: Used in title. Set to 01h by $0CA3 and game mode Ah

D069: Enemy passable block threshold

D06E: Highest OAM stack pointer

D073: Two byte pointer, data for [$C215] before subtracting 21h; pointer to SRAM? Compared with F0h to enable $D09F, probably credits text

D076: Used in v-blank handler. Flag to go to $5:403D; set every few frames during credits
D077: Acid damage. Saved to SRAM
D078: Spike damage. Saved to SRAM
D079: Flag to load characters. But also used in facing screen as a flag to check if buttons are pressed
D07A: Save slot option selected
        0: Start
        1: Clear
D07B: Used in title. Y position of stars?
D07C: Used in title. X position of stars?

D07E: BG palette
D07F: Object 0 palette
D080: Object 1 palette
D081: Samus' max missiles(upper nybble tens, lower nybble ones), mirrored from $D81A at save load.
D082: Samus' max missiles(upper nybble thousands, lower nybble hundreds), mirrored from $D81B at save load.
D083: Read in $1:7A34, adjusts sprite Y position in $1:4DDF
D084: Samus' health for display,   mirror of $D051/D818?
D086: Samus' missiles for display, mirror of $D053/$D81C?

D089: Real number of Metroids remaining
D08A: Projectile passable block threshold
D08B: Metroid Queen's room flag
        11h: In Metroid Queen's room (set by screen transition command 8)

D08D: Value for $D05D in $31F1
D08E: Index of screen transition command set.
        Set to [$4300 + ([screen Y position high] * 10h + [screen X position high]) * 2] & ~800h by set up door transition
D090: Metroid Queen eating pose
        Sets Samus pose = escaping Metroid Queen when 7, checked for 5/20h and set to 6 in in Metroid Queen's mouth
        0: Otherwise
        1: Samus entering mouth
        2: Mouth closing
        3: Mouth closed
        5: Samus escaping mouth
        6: Swallowing Samus
        7: 
        8: Samus escaping stomach
        10h: Paralysed (can enter mouth)
        20h: 
        22h: Dying
D091: Set to 3 in $8:7EBC if killed a target number of Metroids or 1 if only Metroid Queen is remaining
D092: Song for room

D096: Metroids remaining shuffle timer
D097: Index for $5:5620 jump table (first call in credits)
D098: In-game timer, minutes
D099: In-game timer, hours
D09A: Number of Metroids remaining
D09B: Fade in timer

D09E: Checked and cleared in $2:4000, value for $FFC9 in $0C37
D09F: Flag to display in-game time in credits
D0A0: Debug flag

D0A2: In-game timer, 256-frame periods (roughly 14ths of a minute)
D0A3: Save slot
D0A4: Show clear save slot option flag

D0A7: Metroids remaining in area

D600-1F: OAM data for the starry background in the credits.

D700-3F: Screen transition commands

D800-25: Save data. Data loaded from $1:4E64-89 by game mode Bh, loaded from $A008-2D + save slot * 40h by game mode Ch
    D800: Samus' Y position
    D802: Samus' X position
    D804: Screen Y position
    D806: Screen X position
    D808: Enemy tiles source address (2 bytes)
    D80A: Background tiles source bank
    D80B: Background tiles source address (2 bytes)
    D80D: Metatile definitions source address (bank 8, 2 bytes)
    D80F: Source address of data for $DC00-FF (bank 8, 2 bytes)
    D811: Bank for current room
    D812: Samus passable block threshold (>= this tile index is passable, < is impassible)
    D813: Enemy passable block threshold
    D814: Projectile passable block threshold
    D815: Samus' equipment at save load.
    D816: Samus' beam at save load.
    D817: Samus' max energy tanks at save load.
    D818: Samus' health at save load.
    D819: Samus' filled energy tanks at save load.
    D81A: Samus' max missiles at save load. Upper nybble tens, lower nybble ones.
    D81B: Samus' max missiles at save load. Upper nybble thousands, lower nybble hundreds.
    D81C: Samus' missiles at save load. Upper nybble tens, lower nybble ones.
    D81D: Samus' missiles at save load. Upper nybble thousands, lower nybble hundreds.
    D81E: Direction Samus is facing at save load.
    D81F: Acid damage
    D820: Spike damage
    D821: Real number of Metroids remaining
    D822: Song for room
    D823: In-game timer, minutes
    D824: In-game timer, hours
    D825: Number of Metroids remaining

D900-FF: 10h byte slots? The first byte of which each is cleared when saving
DA00-DBFF: Metatile definitions
DC00-FF: Tile properties
    1: Viscous. Reduces jumping and detaches spider ball on movement. Also causes morph ball sound effect glitch.
    2: Jump-through block. Passable when jumping, impassable in all other states (in all directions).
    4: Fall-through block. Passable when falling, impassable in all other states (in all directions).
    8: Spike
    10h: Acid
    20h: Destructible by shot/bomb
    40h: Destructible by bomb
    80h: Save pillar

DD00-FF: Projectile RAM
    Beams - Takes up three 16-byte rows for each shot on-screen. Spazer and Plasma take up all three immediately.
    DD00, DD10, DD20: Beam state.
        0-4: On-screen, being fired. Value will match the currently-equipped beam's as stored at D055.
        FFh: Not on-screen.
    DD01, DD11, DD21: Direction of fire.
        1: To the right.
        2: to the left.
        4: Upwards.
        8: Downwards.
    DD02, DD12, DD22: Shot Y position.
    DD03, DD13, DD23: Shot X position.
    DD04, DD14, DD24: Unknown, switches between 0 and 8 on a seemingly random interval between firings.
    DD05, DD15, DD25: Displacement from position fired at, ends up differently for each direction.
    
    Missiles - Shares a 16-byte row with the third beam slot.
    DD20: Missile state.
        8: On-screen, being fired.
        FFh: Not on-screen.
    DD21: Direction of fire.
        1: To the right.
        2: to the left.
        4: Upwards.
        8: Downwards.
    DD22: Shot Y position.
    DD23: Shot X position.
    DD24: Unknown, switches between 0 and 8 on a seemingly random interval between firings.
    DD25: Displacement from position fired at, ends up differently for each direction.
    
    Bombs - Takes up three 16-byte rows for each bomb on-screen.
    DD30, DD40, DD50: Bomb state.
        1: Laid, waiting to explode.
        2: Exploding.
        FFh: No bomb on-screen.
    DD31, DD41, DD51: Bomb timer. Starts at the value it was set at and decreases to zero.
    DD32, DD42, DD52: Bomb Y position.
    DD33, DD43, DD53: Bomb X position.
    
DE00-FF: Metatile update entries
    + 0: Dest. address. $0000 terminates update
    + 2: Top-left tile
    + 3: Top-right tile
    + 4: Bottom-left tile
    + 5: Bottom-right tile
DF00-FF: Stack
</code>

=== HRAM ===
<code>
FF80: Buttons
        01: A
        02: B
        04: Select
        08: Start
        10: Right
        20: Left
        40: Up
        80: Down
FF81: New buttons
FF82: V-blank handled flag

FF8D: OAM stack pointer

FF97: Frame counter
FF98: Sprite tile number (see $5:4015). Mirror of sprite Y position (see $30BB). Door scroll flags (see $08FE): [$4200 + [screen Y position, screen] * 16 + [screen X position, screen]]
FF99: Two sprite tile numbers (see $5:4000). Mirror of sprite X position (see $30BB). Door thing (see $08FE): [Samus' Y position] - [$FFC8] + 60h

FF9B: Game mode
        0: Boot
        1: Title screen
        2: Loading save 2
        3: Loading save 3
        4: In-game
        5: Dead
        6: Dying
        7: Game over
        8: Paused
        9: Save to SRAM
        Ah: Unused
        Bh: Start new game
        Ch: Load from save
        Dh: RET
        Eh: RET
        Fh: Unused. Identical to 7, set by game mode Ah
        10h: Unused. Game cleared
        11h: Unused. Functionally identical to 7, set by game mode 10h
        12h: Reached the gunship
        13h: Credits

FFA0-A9: OAM DMA routine
FFAA: VRAM tilemap metatile update address. $FFAA = $9800 + ([row block to update] * 32 + [column block to update]) * 2
FFAC: Index of screen for metatile update. $FFAC = [row screen to update] * 16 + [column screen to update]
FFAD: Index of block for metatile update. $FFAD = [row block to update] * 16 + [column block to update]
FFAE: Number of blocks to update
FFAF: Stack pointer for metatile update entries
FFB1: VRAM DMA source address
FFB3: VRAM DMA dest address, also source offset from $CE20 when [$D08C] is set
FFB5: VRAM DMA size
FFB7-BB: Energy tank graphics, other stuff too though
FFB7: Offset for interval for [$FF98] in $30EA
FFB8: Offset for interval for [$FF99] in $30EA
FFB9: Table index for offset data     in $30EA
FFBA: Start   of interval for [$FF98] in $30EA
FFBB: End     of interval for [$FF98] in $30EA
FFBC: Start   of interval for [$FF99] in $30EA
FFBD: End     of interval for [$FF99] in $30EA

FFBF: Flags to negate values in $30EA
FFC0: Samus' Y position
FFC2: Samus' X position
FFC4: Sprite Y position (see $5:4015)
FFC5: Sprite X position (see $5:4015)
FFC6: Sprite set
FFC7: Sprite attributes (see $5:4015)
FFC8: Screen Y position
FFCA: Screen X position
FFCC: Row to update    (in pixels)
FFCE: Column to update (in pixels)

FFE0-FFF5: Used by active enemies (??)

FFFE: Counts every other in-game frame
</code>