RAM Use

There are two banks of memory that map to 4000 - 4FFF (4K each). The lower bit of $5000 controls which bank is active. Write a 0 for the 1st bank and a 1 for the 2nd bank. The Phoenix game only uses 3K from each bank (4000 - 4BFF). I'm guessing this allows the cabinet to leave out a 1K RAM chip for each bank to save money.

These banks include the video memory with a twist -- literally. When bank 1 is selected, the screen is rotated to face player 2 in cocktail mode. When bank 0 is selected, the screen faces player 1 (and both players in a non-cocktail cabinet).

The idea for these banks is that the first bank holds all the player 1 info and the second holds all the player 2 info. This could make switching players easy. But there is no "common" memory for global info and the stack. The code must carefully manage the bank switching -- especially in regards to the stack pointer.

- 4000 - 433F Foreground - 4340 - 47FF General storage (see the table below) - 4800 - 4B3F Background - 4B40 - 4BFF Stack space

Screen memory

The screen is rotated physically clockwise, but the screen memory layout is standard upper-left corner to lower right corner. There are two screens: foreground and background. Each screen is 26 columns by 32 rows (after rotation).

The first (upper left) byte of screen memory maps to the upper right corner of the rotated screen. Adding one to a screen memory pointer moves 1 row down the screen. Subtracting one moves the pointer 1 row up on the screen. Adding 32 to a screen memory pointer moves 1 column left. Subtracting 32 moves the pointer 1 column right. I spell these out because I get confused easily!

Each graphics tile is 8x8 pixels. This gives a rotated screen dimension of 26*8 x 32*8 = 208x256 pixels.

Variables

The values below are kept in bank 0. ?? TODO see how/if the 2nd bank is used? Maybe it is just a copy in cocktail?

Foreground screen

4000:433F	ForegroundScreen	32*26 bytes for the foreground screen

General storage

Alien-formation attack controller data

Drives when and how aliens break out of the base formation to fly their "closed loop" swooping attack patterns. The whole structure (`$4350`–`$437F`) is zeroed at level init by the routine at `$32B0`, and it's serviced every frame by a 7 way state machine dispatched through the jump table `T3018` (indexed by `Counter93`), with each handler reading/updating the behavior state in `$4350`.

4350	M4350	Alien behavior state (the state-machine variable, values 0–6)
4351	M4351	MSB of next closed loop pattern (into the `T2Exx` / `T3330` pattern tables)
4352	M4352	LSB of next closed loop pattern (into the `T2Exx` / `T3330` pattern tables)
4353	M4353	Number of aliens doing the closed loop pattern (how many attackers)
4354	M4354	LSB pointer to the currently selected lead attacking alien (`$4B50`/`$4B72` grid slot)
4355	M4355	Delay counter before the next attack is armed (→ behavior state 1)
4356	M4356	Rotating 0–15 "movement start" index (from `$4395`) used to sync aliens
4357	M4357	Attack-cycle/escalation counter (0–3); scales attacker count and difficulty
4358	M4358	Inter-step timer for the angry downward-push movement pattern
4359	M4359	Staggered group countdown timer 1 for phased alien launches
435A	M435A	Staggered group countdown timer 2 for phased alien launches
435B	M435B	Staggered group countdown timer 3 for phased alien launches
435C	M435C	Reserved/unused byte
435D	M435D	Reserved/unused byte

Flags and counter

435E	M435E	Flag for: 'AliensLeft < 5' ($FF)
435F	M435F	8 bit counter for alien movement
4360	PlayerMoved	Flag for: 'Player moved' ($FF)
4361	BulletTriggered	Flag for: 'Bullet triggered' ($30) and counter
4362	M4362	Flag for: 'Player shield active' and animation counter
4363	ParticleExplosion	Flag for: 'Particle explosion start' and animation counter
4364	M4364	Flag for: 'Enemy hit detected' ($FF) and counter
4365	M4365	Reserved/unused byte
4366	M4366	Flag for: 'Mothership or bird wing hit detected' ($FF)
4367	M4367	Flag for: 'Mothership partially faded in' ($FF)
4368	M4368	Maturity of the birds. From 'egg' over 'no wings' to 'adult' ($01 to $0F)
4369	M4369	Flag for: 'Bonus explosion' ($FF)
436A	M436A	Flag for: 'Bonus live added' ($FF) and counter
436B	M436B	Flag for: 'Mother ship score display' ($FF) and counter
436C	M436C	Reserved/unused byte

Shape of the next bird attack

All three are recomputed by `L3560` each time a new bird group is dispatched.

436D	M436D	Horizontal start position of the bird group
436E	M436E	Bird count / formation-size for the wave
436F	M436F	Per-wave random variation seed (keeps successive waves from being identical)

Explosion slots for animation

4370	M4370	Explosion slot0 animation index
4371	M4371	Explosion slot0 BCD score value (last digit is ever 0)
4372	M4372	Explosion slot0 MSB screen ram
4373	M4373	Explosion slot0 LSB screen ram
4374	M4374	Explosion slot1 animation index
4375	M4375	Explosion slot1 BCD score value (last digit is ever 0)
4376	M4376	Explosion slot1 MSB screen ram
4377	M4377	Explosion slot1 LSB screen ram
4378	M4378	Bonus explosion slot0 animation index
4379	M4379	Bonus explosion slot0 BCD score value (last digit is ever 0)
437A	M437A	Bonus explosion slot0 MSB screen ram
437B	M437B	Bonus explosion slot0 LSB screen ram
437C	M437C	Bonus explosion slot1 animation index
437D	M437D	Bonus explosion slot1 BCD score value (last digit is ever 0)
437E	M437E	Bonus explosion slot1 MSB screen ram
437F	M437F	Bonus explosion slot1 LSB screen ram

For the score

4380	M4380	Ever set to 0 (prevents overflow)
4381	Score1high	Player 1 score BCD (high)
4382	Score1mid	Player 1 score BCD (mid)
4383	Score1low	Player 1 score BCD (low)
4384	M4384	Ever set to 0 (prevents overflow)
4385	Score2high	Player 2 score BCD (high)
4386	Score2mid	Player 2 score BCD (mid)
4387	Score2low	Player 2 score BCD (low)
4388	M4388	Ever set to 0 (prevents overflow)
4389	HiScorehigh	Hi score BCD (high)
438A	HiScoremid	Hi score BCD (mid)
438B	HiScorelow	Hi score BCD (low)

For general purposes

438C	SoundControlA	RAM copy of sound device control register A (0x6000)
438D	SoundControlB	RAM copy of sound device control register B (0x6800)
438E	M438E	Bird-wave background-sound phase (advances the melody, low bit drives SoundControlB)
438F	CoinCount	Number of coins inserted (max is 9)
4390	Player1Lives	Player 1 number of lives
4391	Player2Lives	Player 2 number of lives
4392	M4392	Ever set to 0
4393	Counter93	Free running counter during playtime at game level 3
4394	M4394	Start value list pointer for alien movement MSB
4395	M4395	Start value list pointer for alien movement LSB
4396	M4396	Bird-wave background-sound step timer (counts frames per tone vs `T3DE0` duration)
4397	M4397	Score "dirty" flag — `0` means score changed this frame -> redraw the digits
4398:4399	Counter98	16 bit counter (MSB:LSB) actual index for slow print at intro splash
439A:439B	Counter9A	16 bit counter (MSB:LSB) and..
439B	M439B	Next index for slow print at intro splash
439C	M439C	Spiral-fill animation step counter (level 4/6/8 inter-wave fade-in)
439D	M439D	Fist two digits of BCD score value for mothership explosion
439E	M439E	Mapped player ship position, left part: ($09 to $C0)
439F	M439F	Mapped player ship position, right part: ($17 to $C8)
43A0	IN0Current	Current value of IN0: bit0='coin', bit1='1 player', bit2='2 players', bit4='fire', bit5='right', bit6='left', bit7='shield'
43A1	IN0Previous	Previous value of IN0
43A2	GameOrAttract	Attract mode=0, One player game mode=1, Two players game mode=2
43A3	GameAndDemoOrSplash	Game and demo for player 1=0, Game for player 2=1, Intro splash=2
43A4	GameState	Game state=0 - 7
43A5	CounterA5	8 bit counter (e.g.: score flash time)
43A6	ShieldCount	Counts shield time and controls shield picture. Shields end at C0.
43A7	AnimationCounter	For mothership's antenna and the alien pilot animation
43A8	M43A8	Temporary storage (MSB of pointer to table $1860)
43A9	M43A9	Temporary storage (LSB of pointer to table $1860)
43AA	M43AA	Mothership-wave frame counter (times antenna/pilot animation and star scroll)
43AB	M43AB	Counter for planet trigger
43AC	M43AC	Planet vertical spacing increment (added to trigger `$43AB`)
43AD	M43AD	Planet X index -> `T1E60` (screen-RAM LSB), incremented per planet
43AE	M43AE	Planet X index -> `T1E20` (screen-RAM MSB), incremented per planet
43AF	M43AF	`CounterB9` trigger for the next galaxy
43B0	M43B0	Galaxy spacing decrement (subtracted from `$43AF`)
43B1	M43B1	Galaxy X index -> `T1E80`, incremented per galaxy
43B2	M43B2	MSB pointer into the background pattern tables `T1C00`/`T1D00`/`T1F00`
43B3	M43B3	LSB pointer into the background pattern tables `T1C00`/`T1D00`/`T1F00`
43B4	CounterB4	8 bit counter (stars scrolling down, aliens fade in time)
43B5	M43B5	Reserved/unused byte
43B6	M43B6	End-of-wave countdown timer that advances the game to the next level/round
43B7	M43B7	Reserved/unused byte
43B8	LevelAndRound	Bit0 - 3: game level, bit4 - 7: game round
43B9	CounterB9	8 bit backwards counter
43BA	AliensLeft	Number of aliens left in wave (16 at new)
43BB	BirdsLeft	Number of birds left in wave (8 at new)
43BC	M43BC	Reserved/unused byte
43BD	M43BD	Low byte of the bonus extra-life score threshold; rewritten (nibble-swapped `BonusLivesAt`) after a bonus is granted
43BE	BonusLivesAt	Middle byte of the threshold, set from DIP switches to `$30/$40/$50/$60` = 3000/4000/5000/6000 points
43BF	M43BF	High byte of the bonus extra-life score threshold

Player and player bullets, data structure (grid)

MAME cheat code "Infinite Shields": set $84 (%1000_0100) at $43C0

43C0	PlayerState	Player ship control state register
43C1	PlayerShape	LSB for T1400 player ship character block shapes table
43C2	PlayerShipX	Player ship, coordinate X ($0C=min.left, $64=default, $C0=max.right)
43C3	PlayerShipY	Player ship, coordinate Y ($D8)
43C4	PlayerBulletState	Player bullet, control state register
43C5	PlayerBulletShape	Player bullet, character code ($50 to $57)
43C6	PlayerBulletX	Player bullet, coordinate X
43C7	PlayerBulletY	Player bullet, coordinate Y ($D0=min.bottom, $18=max.top)
43C8	AbovePlayerBulletState	One position above player bullet, control state register
43C9	AbovePlayerBulletShape	One position above player bullet, character code ($50 to $57)
43CA	AbovePlayerBulletX	One position above player bullet, coordinate X
43CB	AbovePlayerBulletY	One position above player bullet, coordinate Y

Alien and bird bullets, data structure (grid)

43CC	EnemyBullet0State	Enemy bullet 0, control state register
43CD	EnemyBullet0Shape	Enemy bullet 0, character code ($58 to $5F)
43CE	EnemyBullet0X	Enemy bullet 0, coordinate X
43CF	EnemyBullet0Y	Enemy bullet 0, coordinate Y
43D0	EnemyBullet1State	Enemy bullet 1, control state register
43D1	EnemyBullet1Shape	Enemy bullet 1, character code ($58 to $5F)
43D2	EnemyBullet1X	Enemy bullet 1, coordinate X
43D3	EnemyBullet1Y	Enemy bullet 1, coordinate Y
43D4	EnemyBullet2State	Enemy bullet 2, control state register
43D5	EnemyBullet2Shape	Enemy bullet 2, character code ($58 to $5F)
43D6	EnemyBullet2X	Enemy bullet 2, coordinate X
43D7	EnemyBullet2Y	Enemy bullet 2, coordinate Y
43D8	EnemyBullet3State	Enemy bullet 3, control state register
43D9	EnemyBullet3Shape	Enemy bullet 3, character code ($58 to $5F)
43DA	EnemyBullet3X	Enemy bullet 3, coordinate X
43DB	EnemyBullet3Y	Enemy bullet 3, coordinate Y
43DC	EnemyBullet4State	Enemy bullet 4, control state register
43DD	EnemyBullet4Shape	Enemy bullet 4, character code ($58 to $5F)
43DE	EnemyBullet4X	Enemy bullet 4, coordinate X
43DF	EnemyBullet4Y	Enemy bullet 4, coordinate Y

Player and player bullets, data structure (screen ram)

43E0	OldPlayerShipMSB	Old MSB screen ram: Upper left character of player ship
43E1	OldPlayerShipLSB	Old LSB screen ram: Upper left character of player ship
43E2	PlayerShipMSB	MSB screen ram: Upper left character of player ship
43E3	PlayerShipLSB	LSB screen ram: Upper left character of player ship
43E4	PlayerBulletMSB	MSB screen ram: Player bullet
43E5	PlayerBulletLSB	LSB screen ram: Player bullet
43E6	AbovePlayerBulletMSB	MSB screen ram: One character above player bullet
43E7	AbovePlayerBulletLSB	LSB screen ram: One character above player bullet
43E8	M43E8	MSB of its previous position (erase pointer, refreshed by `L0886`)
43E9	M43E9	LSB of its previous position (erase pointer, refreshed by `L0886`)
43EA	M43EA	MSB of its current position (draw + collision pointer, recomputed each frame by `L09A0` from `$43CA:$43CB`)
43EB	M43EB	LSB of its current position (draw + collision pointer, recomputed each frame by `L09A0` from `$43CA:$43CB`)

Alien and bird bullets, data structure (screen ram)

43EC	OldEnemyBullet0MSB	Old MSB screen ram: Enemy bullet 0
43ED	OldEnemyBullet0LSB	Old LSB screen ram: Enemy bullet 0
43EE	EnemyBullet0MSB	MSB screen ram: Enemy bullet 0
43EF	EnemyBullet0LSB	LSB screen ram: Enemy bullet 0
43F0	OldEnemyBullet1MSB	Old MSB screen ram: Enemy bullet 1
43F1	OldEnemyBullet1LSB	Old LSB screen ram: Enemy bullet 1
43F2	EnemyBullet1MSB	MSB screen ram: Enemy bullet 1
43F3	EnemyBullet1LSB	LSB screen ram: Enemy bullet 1
43F4	OldEnemyBullet2MSB	Old MSB screen ram: Enemy bullet 2
43F5	OldEnemyBullet2LSB	Old LSB screen ram: Enemy bullet 2
43F6	EnemyBullet2MSB	MSB screen ram: Enemy bullet 2
43F7	EnemyBullet2LSB	LSB screen ram: Enemy bullet 2
43F8	OldEnemyBullet3MSB	Old MSB screen ram: Enemy bullet 3
43F9	OldEnemyBullet3LSB	Old LSB screen ram: Enemy bullet 3
43FA	EnemyBullet3MSB	MSB screen ram: Enemy bullet 3
43FB	EnemyBullet3LSB	LSB screen ram: Enemy bullet 3
43FC	OldEnemyBullet4MSB	Old MSB screen ram: Enemy bullet 4
43FD	OldEnemyBullet4LSB	Old LSB screen ram: Enemy bullet 4
43FE	EnemyBullet4MSB	MSB screen ram: Enemy bullet 4
43FF	EnemyBullet4LSB	LSB screen ram: Enemy bullet 4

Background screen

4800:4B3F	BackgroundScreen	32*26 bytes for the background screen

Pointer to alien movement pattern

Default values (all: $10,$00 pointing to T1000) are defined at table: T1520.

4B50	M4B50	Alien0 movement pattern table MSB
4B51	M4B51	Alien0 movement pattern table LSB
4B52	M4B52	Alien1 movement pattern table MSB
4B53	M4B53	Alien1 movement pattern table LSB
4B54	M4B54	Alien2 movement pattern table MSB
4B55	M4B55	Alien2 movement pattern table LSB
4B56	M4B56	Alien3 movement pattern table MSB
4B57	M4B57	Alien3 movement pattern table LSB
4B58	M4B58	Alien4 movement pattern table MSB
4B59	M4B59	Alien4 movement pattern table LSB
4B5A	M4B5A	Alien5 movement pattern table MSB
4B5B	M4B5B	Alien5 movement pattern table LSB
4B5C	M4B5C	Alien6 movement pattern table MSB
4B5D	M4B5D	Alien6 movement pattern table LSB
4B5E	M4B5E	Alien7 movement pattern table MSB
4B5F	M4B5F	Alien7 movement pattern table LSB
4B60	M4B60	Alien8 movement pattern table MSB
4B61	M4B61	Alien8 movement pattern table LSB
4B62	M4B62	Alien9 movement pattern table MSB
4B63	M4B63	Alien9 movement pattern table LSB
4B64	M4B64	AlienA movement pattern table MSB
4B65	M4B65	AlienA movement pattern table LSB
4B66	M4B66	AlienB movement pattern table MSB
4B67	M4B67	AlienB movement pattern table LSB
4B68	M4B68	AlienC movement pattern table MSB
4B69	M4B69	AlienC movement pattern table LSB
4B6A	M4B6A	AlienD movement pattern table MSB
4B6B	M4B6B	AlienD movement pattern table LSB
4B6C	M4B6C	AlienE movement pattern table MSB
4B6D	M4B6D	AlienE movement pattern table LSB
4B6E	M4B6E	AlienF movement pattern table MSB
4B6F	M4B6F	AlienF movement pattern table LSB

Alien data structure (grid)

Used for all levels with the 16 aliens. Level: 1, 2, 5 (with mothership), 6, 7, 10(with mothership). During 'fade in' phase, the alien control state B is holding the character code!

4B70	M4B70	Alien0 control state A
4B71	M4B71	Alien0 control state B (LSB for T14xx)
4B72	M4B72	Alien0 screen coordinate X
4B73	M4B73	Alien0 screen coordinate Y
4B74	M4B74	Alien1 control state A
4B75	M4B75	Alien1 control state B (LSB for T14xx)
4B76	M4B76	Alien1 screen coordinate X
4B77	M4B77	Alien1 screen coordinate Y
4B78	M4B78	Alien2 control state A
4B79	M4B79	Alien2 control state B (LSB for T14xx)
4B7A	M4B7A	Alien2 screen coordinate X
4B7B	M4B7B	Alien2 screen coordinate Y
4B7C	M4B7C	Alien3 control state A
4B7D	M4B7D	Alien3 control state B (LSB for T14xx)
4B7E	M4B7E	Alien3 screen coordinate X
4B7F	M4B7F	Alien3 screen coordinate Y
4B80	M4B80	Alien4 control state A
4B81	M4B81	Alien4 control state B (LSB for T14xx)
4B82	M4B82	Alien4 screen coordinate X
4B83	M4B83	Alien4 screen coordinate Y
4B84	M4B84	Alien5 control state A
4B85	M4B85	Alien5 control state B (LSB for T14xx)
4B86	M4B86	Alien5 screen coordinate X
4B87	M4B87	Alien5 screen coordinate Y
4B88	M4B88	Alien6 control state A
4B89	M4B89	Alien6 control state B (LSB for T14xx)
4B8A	M4B8A	Alien6 screen coordinate X
4B8B	M4B8B	Alien6 screen coordinate Y
4B8C	M4B8C	Alien7 control state A
4B8D	M4B8D	Alien7 control state B (LSB for T14xx)
4B8E	M4B8E	Alien7 screen coordinate X
4B8F	M4B8F	Alien7 screen coordinate Y
4B90	M4B90	Alien8 control state A
4B91	M4B91	Alien8 control state B (LSB for T14xx)
4B92	M4B92	Alien8 screen coordinate X
4B93	M4B93	Alien8 screen coordinate Y
4B94	M4B94	Alien9 control state A
4B95	M4B95	Alien9 control state B (LSB for T14xx)
4B96	M4B96	Alien9 screen coordinate X
4B97	M4B97	Alien9 screen coordinate Y
4B98	M4B98	AlienA control state A
4B99	M4B99	AlienA control state B (LSB for T14xx)
4B9A	M4B9A	AlienA screen coordinate X
4B9B	M4B9B	AlienA screen coordinate Y
4B9C	M4B9C	AlienB control state A
4B9D	M4B9D	AlienB control state B (LSB for T14xx)
4B9E	M4B9E	AlienB screen coordinate X
4B9F	M4B9F	AlienB screen coordinate Y
4BA0	M4BA0	AlienC control state A
4BA1	M4BA1	AlienC control state B (LSB for T14xx)
4BA2	M4BA2	AlienC screen coordinate X
4BA3	M4BA3	AlienC screen coordinate Y
4BA4	M4BA4	AlienD control state A
4BA5	M4BA5	AlienD control state B (LSB for T14xx)
4BA6	M4BA6	AlienD screen coordinate X
4BA7	M4BA7	AlienD screen coordinate Y
4BA8	M4BA8	AlienE control state A
4BA9	M4BA9	AlienE control state B (LSB for T14xx)
4BAA	M4BAA	AlienE screen coordinate X
4BAB	M4BAB	AlienE screen coordinate Y
4BAC	M4BAC	AlienF control state A
4BAD	M4BAD	AlienF control state B (LSB for T14xx)
4BAE	M4BAE	AlienF screen coordinate X
4BAF	M4BAF	AlienF screen coordinate Y

Bird object data structure

Used for all levels with the 8 birds. Level: 3, 4, 8, 9.

For each bird object in `$4B70`–`$4BAF`:

(+3) — Animation phase / current shape frame: Selects which frame of the bird's wing-flap/animation group is drawn. It's advanced by the movement step each update (see below), wrapping mod 8, which cycles the animation. At the intro it's borrowed as scratch — `$4B70`–`$4B73` are reused to draw a single bird frame, with `$4B73 = index & 7`, the animation phase: `$21DF` "used as temp memory".

(+4) — Movement-step countdown timer: On each bird update (`L35B0`), the code steps to +4 and decrements it (unless already 0).

(+6) — horizontal movement step (velocity): Added each update to the grid X position (`$4B75`) and to the animation phase (`$4B73`); `≥ $10` triggers special wing-handling.

4B70	B4B70	Bird0 character-block shape index
4B71	B4B71	Bird0 screen-RAM address MSB
4B72	B4B72	Bird0 screen-RAM address LSB
4B73	B4B73	Bird0 animation phase / current shape frame
4B74	B4B74	Bird0 movement-step countdown timer
4B75	B4B75	Bird0 grid coordinate X
4B76	B4B76	Bird0 horizontal movement step (velocity)
4B77	B4B77	Bird0 grid coordinate Y
4B78	B4B78	Bird1 character-block shape index
4B79	B4B79	Bird1 screen-RAM address MSB
4B7A	B4B7A	Bird1 screen-RAM address LSB
4B7B	B4B7B	Bird1 animation phase / current shape frame
4B7C	B4B7C	Bird1 movement-step countdown timer
4B7D	B4B7D	Bird1 grid coordinate X
4B7E	B4B7E	Bird1 horizontal movement step (velocity)
4B7F	B4B7F	Bird1 grid coordinate Y
4B80	B4B80	Bird2 character-block shape index
4B81	B4B81	Bird2 screen-RAM address MSB
4B82	B4B82	Bird2 screen-RAM address LSB
4B83	B4B83	Bird2 animation phase / current shape frame
4B84	B4B84	Bird2 movement-step countdown timer
4B85	B4B85	Bird2 grid coordinate X
4B86	B4B86	Bird2 horizontal movement step (velocity)
4B87	B4B87	Bird2 grid coordinate Y
4B88	B4B88	Bird3 character-block shape index
4B89	B4B89	Bird3 screen-RAM address MSB
4B8A	B4B8A	Bird3 screen-RAM address LSB
4B8B	B4B8B	Bird3 animation phase / current shape frame
4B8C	B4B8C	Bird3 movement-step countdown timer
4B8D	B4B8D	Bird3 grid coordinate X
4B8E	B4B8E	Bird3 horizontal movement step (velocity)
4B8F	B4B8F	Bird3 grid coordinate Y
4B90	B4B90	Bird4 character-block shape index
4B91	B4B91	Bird4 screen-RAM address MSB
4B92	B4B92	Bird4 screen-RAM address LSB
4B93	B4B93	Bird4 animation phase / current shape frame
4B94	B4B94	Bird4 movement-step countdown timer
4B95	B4B95	Bird4 grid coordinate X
4B96	B4B96	Bird4 horizontal movement step (velocity)
4B97	B4B97	Bird4 grid coordinate Y
4B98	B4B98	Bird5 character-block shape index
4B99	B4B99	Bird5 screen-RAM address MSB
4B9A	B4B9A	Bird5 screen-RAM address LSB
4B9B	B4B9B	Bird5 animation phase / current shape frame
4B9C	B4B9C	Bird5 movement-step countdown timer
4B9D	B4B9D	Bird5 grid coordinate X
4B9E	B4B9E	Bird5 horizontal movement step (velocity)
4B9F	B4B9F	Bird5 grid coordinate Y
4BA0	B4BA0	Bird6 character-block shape index
4BA1	B4BA1	Bird6 screen-RAM address MSB
4BA2	B4BA2	Bird6 screen-RAM address LSB
4BA3	B4BA3	Bird6 animation phase / current shape frame
4BA4	B4BA4	Bird6 movement-step countdown timer
4BA5	B4BA5	Bird6 grid coordinate X
4BA6	B4BA6	Bird6 horizontal movement step (velocity)
4BA7	B4BA7	Bird6 grid coordinate Y
4BA8	B4BA8	Bird7 character-block shape index
4BA9	B4BA9	Bird7 screen-RAM address MSB
4BAA	B4BAA	Bird7 screen-RAM address LSB
4BAB	B4BAB	Bird7 animation phase / current shape frame
4BAC	B4BAC	Bird7 movement-step countdown timer
4BAD	B4BAD	Bird7 grid coordinate X
4BAE	B4BAE	Bird7 horizontal movement step (velocity)
4BAF	B4BAF	Bird7 grid coordinate Y

Alien data structure (screen ram)

4BB0	M4BB0	Old MSB screen ram adress alien0
4BB1	M4BB1	Old LSB screen ram adress alien0
4BB2	M4BB2	MSB screen ram adress alien0
4BB3	M4BB3	LSB screen ram adress alien0
4BB4	M4BB4	Old MSB screen ram adress alien1
4BB5	M4BB5	Old LSB screen ram adress alien1
4BB6	M4BB6	MSB screen ram adress alien1
4BB7	M4BB7	LSB screen ram adress alien1
4BB8	M4BB8	Old MSB screen ram adress alien2
4BB9	M4BB9	Old LSB screen ram adress alien2
4BBA	M4BBA	MSB screen ram adress alien2
4BBB	M4BBB	LSB screen ram adress alien2
4BBC	M4BBC	Old MSB screen ram adress alien3
4BBD	M4BBD	Old LSB screen ram adress alien3
4BBE	M4BBE	MSB screen ram adress alien3
4BBF	M4BBF	LSB screen ram adress alien3
4BC0	M4BC0	Old MSB screen ram adress alien4
4BC1	M4BC1	Old LSB screen ram adress alien4
4BC2	M4BC2	MSB screen ram adress alien4
4BC3	M4BC3	LSB screen ram adress alien4
4BC4	M4BC4	Old MSB screen ram adress alien5
4BC5	M4BC5	Old LSB screen ram adress alien5
4BC6	M4BC6	MSB screen ram adress alien5
4BC7	M4BC7	LSB screen ram adress alien5
4BC8	M4BC8	Old MSB screen ram adress alien6
4BC9	M4BC9	Old LSB screen ram adress alien6
4BCA	M4BCA	MSB screen ram adress alien6
4BCB	M4BCB	LSB screen ram adress alien6
4BCC	M4BCC	Old MSB screen ram adress alien7
4BCD	M4BCD	Old LSB screen ram adress alien7
4BCE	M4BCE	MSB screen ram adress alien7
4BCF	M4BCF	LSB screen ram adress alien7
4BD0	M4BD0	Old MSB screen ram adress alien8
4BD1	M4BD1	Old LSB screen ram adress alien8
4BD2	M4BD2	MSB screen ram adress alien8
4BD3	M4BD3	LSB screen ram adress alien8
4BD4	M4BD4	Old MSB screen ram adress alien9
4BD5	M4BD5	Old LSB screen ram adress alien9
4BD6	M4BD6	MSB screen ram adress alien9
4BD7	M4BD7	LSB screen ram adress alien9
4BD8	M4BD8	Old MSB screen ram adress alienA
4BD9	M4BD9	Old LSB screen ram adress alienA
4BDA	M4BDA	MSB screen ram adress alienA
4BDB	M4BDB	LSB screen ram adress alienA
4BDC	M4BDC	Old MSB screen ram adress alienB
4BDD	M4BDD	Old LSB screen ram adress alienB
4BDE	M4BDE	MSB screen ram adress alienB
4BDF	M4BDF	LSB screen ram adress alienB
4BE0	M4BE0	Old MSB screen ram adress alienC
4BE1	M4BE1	Old LSB screen ram adress alienC
4BE2	M4BE2	MSB screen ram adress alienC
4BE3	M4BE3	LSB screen ram adress alienC
4BE4	M4BE4	Old MSB screen ram adress alienD
4BE5	M4BE5	Old LSB screen ram adress alienD
4BE6	M4BE6	MSB screen ram adress alienD
4BE7	M4BE7	LSB screen ram adress alienD
4BE8	M4BE8	Old MSB screen ram adress alienE
4BE9	M4BE9	Old LSB screen ram adress alienE
4BEA	M4BEA	MSB screen ram adress alienE
4BEB	M4BEB	LSB screen ram adress alienE
4BEC	M4BEC	Old MSB screen ram adress alienF
4BED	M4BED	Old LSB screen ram adress alienF
4BEE	M4BEE	MSB screen ram adress alienF
4BEF	M4BEF	LSB screen ram adress alienF

Bird extended storage

Scratch RAM used by the routine `L3980` (`$3980`), which runs during the bird/Phoenix waves to do an extended vertical collision sweep of the player's shot against the descending birds.

Buffer for real bullet/aim values while routine `L39F0` hijacks them (copied back via `L39DB`).

The routine works by hijacking the real player-bullet variables, sweeping them down through several screen rows running the bird collision check at each row, and then putting the originals back. `$4BC0`–`$4BC5` hold the saved originals during that sweep.

4BC0	B4BC0	saved PlayerBulletState
4BC1	B4BC1	saved PlayerBulletShape
4BC2	B4BC2	saved PlayerBulletX
4BC3	B4BC3	saved PlayerBulletY
4BC4	B4BC4	saved AbovePlayerBulletMSB
4BC5	B4BC5	saved AbovePlayerBulletLSB

Bird-wave descent/attack control variables:

They all live in the enemy work-RAM block that's cleared at each level start (`$4B50`–`$4BEF`, `$0535`). The `$4BD0` group is the heart of the swooping-formation logic during the bird (Phoenix) waves.

`$4BD1` — Descent turnaround threshold (max depth):

Computed in `L2476` from remaining birds, the wave timer, and the random seed `$436F`. The formation keeps descending until the scroll phase `$4BD2` passes this value, at which point the motion routine switches to the return/up path.

`$4BD2` — Vertical scroll phase of the bird formation (the master index):

Recomputed every frame from the scroll counter `CounterB9`. It's a 0–31 value representing how far the formation has scrolled down, and it's the index used all over the bird code (collision tables, the `T3ED0` scroll-increment table, the `T3DC0` active-object window, etc.).

`$4BD3` — Countdown timer between bird attacks ("bird extended storage"):

Loaded in `L2476` and decremented each cycle in `L26AA`; when it hits 0 the next attack/movement pattern is armed.

`$4BD4` — Attack sub-pattern selector (0–3):

Set from the low 2 bits of the random seed `$436F`; picks one of four bird attack variants.

`$4BD5` — Descent step / speed value:

Computed in `L2668` (base `$436E`, clamped by the `$3EE0` speed curve and tweaked by `BirdsLeft`), then consumed by the scroll routine: its low 2 bits index the `$3ED0` dither table and `bits>>2` give the coarse pixels-per-frame.

`$4BD6` — Combined scroll-phase + active-bird center index (0–31):

Built in `L26D0` by scanning the live bird objects (tracking the first and last active rows `D`/`E`) and adding them to the scroll phase. It indexes the `$3EE0` clamp curve, the `T3DE0` background-tone-duration table, and the bird-count clamp at `$2685`.

`$4BD7` — Vertical spread of the active birds (computed, effectively unused):

Also produced by `L26D0` as `lastRow − firstRow`. It is written at $26FA but not read anywhere else in the ROM — a leftover/vestigial value (the formation span is computed but only `$4BD6` is actually used downstream).

4BD0	B4BD0	Unused RAM (cleared at level init, never referenced)
4BD1	B4BD1	Descent turnaround depth threshold (formation reverses when `$4BD2` passes it)
4BD2	B4BD2	Vertical scroll phase 0–31 of the bird formation (derived from `CounterB9`; master index) (0..31)
4BD3	B4BD3	Countdown timer between bird attack launches
4BD4	B4BD4	Attack sub-pattern selector (0–3, from random `$436F`) for one of four attack variants
4BD5	B4BD5	Descent step/speed value (feeds the `$3ED0` dither scroll rate)
4BD6	B4BD6	Combined scroll-phase + active-bird center index (indexes `$3EE0` curve & `T3DE0` sound)
4BD7	B4BD7	Active-bird vertical spread — computed but not consumed (vestigial)
4BED	B4BED	Unused RAM (cleared at level init, never referenced)
4BEE	B4BEE	Unused RAM (cleared at level init, never referenced)
4BEF	B4BEF	Unused RAM (cleared at level init, never referenced)

Stack

4BF0:4BFF	Stack	Stack space