RAM Usage

2000	waitOnDraw	Cleared by alien-draw and set by next-alien. This ensures no alien gets missed while drawing.
2001
2002	alienIsExploding	Not-0 if an alien is exploding, 0 if not exploting
2003	expAlienTimer	Time (ISR ticks) left in alien-explosion
2004	alienRow	Row number of current alien (cursor)
2005	alienFrame	Animation frame number (0 or 1) for current alien (cursor)
2006	alienCurIndex	Alien cursor index (from 0 to 54)
2007	refAlienDYr	Reference alien delta Yr
2008	refAlienDXr	Reference alien deltaXr
2009	refAlienYr	Reference alien Yr coordinate
200A	refAlienXr	Reference alien Xr coordinate
200B	alienPosLSB	Alien cursor bit pos (LSB)
200C	alienPosMSB	Alien cursor bit pos (MSB)
200D	rackDirection	Value 0 if rack is moving right or 1 if rack is moving left
200E	rackDownDelta	Constant value of alien rack dropping after bumping screen edge
200F
2010	obj0TimerMSB
2011	obj0TimerLSB	Wait 128 interrupts (about 2 secs) before player task starts
2012	obj0TimerExtra
2013	obj0HanlderLSB
2014	oBJ0HanlderMSB	Player handler code at 028E
2015	playerAlive	Player is alive (FF=alive). Toggles between 0 and 1 for blow-up images.
2016	expAnimateTimer	Time till next blow-up sprite change (reloaded to 5)
2017	expAnimateCnt	Number of changes left in blow-up sequence
2018	plyrSprPicL	Player sprite descriptor ... picture LSB
2019	plyrSprPicM	Player sprite descriptor ... picture MSB
201A	playerYr	Player sprite descriptor ... location LSB
201B	playerXr	Player sprite descriptor ... location MSB
201C	plyrSprSiz	Player sprite descriptor ... size of sprite
201D	nextDemoCmd	Next movement command for demo
201E	hidMessSeq	Set to 1 after 1st of 2 sequences are entered for hidden-message display
201F		Appears to be unused
2020	obj1TimerMSB
2021	obj1TimerLSB
2022	obj1TimerExtra	All 0's ... run immediately
2023	obj1HandlerLSB
2024	obj1HandlerMSB	Shot handler code at 03BB
2025	plyrShotStatus	0 if available, 1 if just initiated, 2 moving normally, 3 hit something besides alien, 5 if alien explosion is in progress, 4 if alien has exploded (remove from active duty)
2026	blowUpTimer	Sprite blow-up timer
2027	obj1ImageLSB
2028	obj1ImageMSB	Sprite image at 1C90 (just one byte)
2029	obj1CoorYr	Player shot Y coordinate
202A	obj1CoorXr	Player shot X coordinate
202B	obj1ImageSize	Size of shot image (just one byte)
202C	shotDeltaX	Shot's delta X
202D	fireBounce	1 if button has been handled but remains down
202E
202F
2030	obj2TimerMSB
2031	obj2TimerLSB
2032	obj2TimerExtra	GO-3 runs when this is 1. GO-4 runs when this is 2. (copied to 2080 in game loop)
2033	obj2HandlerLSB
2034	obj2HandlerMSB	Handler code at 0476
2035	rolShotStatus
2036	rolShotStepCnt
2037	rolShotTrack	A 0 means this shot tracks the player
2038	rolShotCFirLSB	Pointer to column-firing table LSB (not used for targeting)
2039	rolShotCFirMSB	Pointer to column-firing table MSB (not used for MSB counter
203A	rolShotBlowCnt
203B	rolShotImageLSB
203C	rolShotImageMSB
203D	rolShotYr
203E	rolShotXr
203F	rolShotSize
2040	obj3TimerMSB
2041	obj3TimerLSB
2042	obj3TimerExtra
2043	obj3HandlerLSB
2044	obj3HandlerMSB	Handler code at 04B6
2045	pluShotStatus
2046	pluShotStepCnt
2047	pluShotTrack	A 1 means this shot does not track the player
2048	pluShotCFirLSB	Pointer to column-firing table LSB
2049	pluShotCFirMSB	Pointer to column-firing table MSB
204A	pluShotBlowCnt
204B	pluShotImageLSB
204C	pluShotImageMSB
204D	pluShotYr
204E	pluSHotXr
204F	pluShotSize
2050	obj4TimerMSB
2051	obj4TimerLSB
2052	obj4TimerExtra
2053	obj4HandlerLSB
2054	obj4HandlerMSB	Handler code at 0682
2055	squShotStatus
2056	squShotStepCnt
2057	squShotTrack	A 1 means this shot does not track the player
2058	squShotCFirLSB	Pointer to column-firing table LSB
2059	squShotCFirMSB	Pointer to column-firing table MSB
205A	squSHotBlowCnt
205B	squShotImageLSB
205C	squShotImageMSB
205D	squShotYr
205E	squShotXr
205F	squShotSize
2060	endOfTasks	FF marks the end of the tasks list
2061	collision	Set to 1 if sprite-draw detects collision
2062	expAlienLSB
2063	expAlienMSB	Exploding alien picture 1CC0
2064	expAlienYr	Y coordinate of exploding alien
2065	expAlienXr	X coordinate of exploding alien
2066	expAlienSize	Size of exploding alien sprite (16 bytes)
2067	playerDataMSB	Current player's data-pointer MSB (21xx or 22xx)
2068	playerOK	1 means OK, 0 means blowing up
2069	enableAlienFire	1 means aliens can fire, 0 means not
206A	alienFireDelay	Count down till aliens can fire (2069 flag is then set)
206B	oneAlien	1 when only one alien is on screen
206C	temp206C	Holds the value ten ... number of characters in each "=xx POINTS" string but gets set to 18 in mem copy before game.
206D	invaded	Set to 1 when player blows up because rack has reached bottom
206E	skipPlunger	When there is only one alien left this goes to 1 to disable the plunger-shot when it ends
206F
2070	otherShot1	When processing a shot, this holds one of the other shot's info
2071	otherShot2	When processing a shot, this holds one of the other shot's info
2072	vblankStatus	80=screen is being drawn (don't touch), 0=blanking in progress (ok to change)
2073	aShotStatus	Bit 0 set if shot is blowing up, bit 7 set if active
2074	aShotStepCnt	Count of steps made by shot (used for fire reload rate)
2075	aShotTrack	0 if shot tracks player or 1 if it uses the column-fire table
2076	aShotCFirLSB	Pointer to column-firing table LSB
2077	aShotCFirMSB	Pointer to column-firing table MSB
2078	aShotBlowCnt	Alen shot blow up counter. At 3 the explosion is drawn. At 0 it is done.
2079	aShotImageLSB	Alien shot image LSB
207A	aShotImageMSB	Alien shot image MSB
207B	alienShotYr	Alien shot delta Y
207C	alienShotXr	Alien shot delta X
207D	alienShotSize	Alien shot size
207E	alienShotDelta	Alien shot speed. Normally -1 but set to -4 with less than 9 aliens
207F	shotPicEnd	the last picture in the current alien shot animation
2080	shotSync	All 3 shots are synchronized to the GO-2 timer. This is copied from timer in the game loop
2081	tmp2081	Used to hold the remember/restore flag in shield-copy routine
2082	numAliens	Number of aliens on screen
2083	saucerStart	Flag to start saucer (set to 1 when 2091:2092 counts down to 0)
2084	saucerActive	Saucer is on screen (1 means yes)
2085	saucerHit	Saucer has been hit (1 means draw it but don't move it)
2086	saucerHitTime	Hit-sequence timer (explosion drawn at 1F, score drawn at 18)
2087	saucerPriLocLSB	Mystery ship print descriptor ... coordinate LSB
2088	saucerPriLocMSB	Mystery ship print descriptor ... coordinate MSB
2089	saucerPriPicLSB	Mystery ship print descriptor ... message LSB
208A	saucerPriPicMSB	Mystery ship print descriptor ... message MSB
208B	saucerPriSize	Mystery ship print descriptor ... number of characters
208C	saucerDeltaY	Mystery ship delta Y
208D	sauScoreLSB	Pointer into mystery-ship score table (MSB)
208E	sauScoreMSB	Pointer into mystery-ship score table (LSB)
208F	shotCountLSB	Bumped every shot-removal. Saucer's direction is bit 0. (0=2/29, 1=-2/E0)
2090	shotCountMSB	Read as two-bytes with 208F, but never used as such.
2091	tillSaucerLSB
2092	tillSaucerMSB	Count down every game loop. When it reaches 0 saucer is triggerd. Reset to 600.
2093	waitStartLoop	1=in wait-for-start loop, 0=in splash screens
2094	soundPort3	Current status of sound port (out $03)
2095	changeFleetSnd	Set to 1 in ISR if time to change the fleet sound
2096	fleetSndCnt	Delay until next fleet movement tone
2097	fleetSndReload	Reload value for fleet sound counter
2098	soundPort5	Current status of sound port (out $05)
2099	extraHold	Duration counter for extra-ship sound
209A	tilt	1 if tilt handling is in progress
209B	fleetSndHold	Time to hold fleet-sound at each change
209C
209D
209E
209F
20A0
20A1
20A2
20A3
20A4
20A5
20A6
20A7
20A8
20A9
20AA
20AB
20AC
20AD
20AE
20AF
20B0
20B1
20B2
20B3
20B4
20B5
20B6
20B7
20B8
20B9
20BA
20BB
20BC
20BD
20BE
20BF
20C0	isrDelay	Delay counter decremented in ISR
20C1	isrSplashTask	1=In demo, 2=Little-alien and Y, 4=shooting extra 'C'
20C2	splashAnForm	Image form (increments each draw)
20C3	splashDeltaX	Delta X
20C4	splashDeltaY	Delta Y
20C5	splashYr	Y coordinate
20C6	splashXr	X coordinate
20C7	splashImageLSB
20C8	splashImageMSB	Base image 1BA0 (small alien with upside down Y)
20C9	splashImageSize	Size of image (16 bytes)
20CA	splashTargetY	Target Y coordinate
20CB	splashReached	Reached target Y flag (1 when reached)
20CC	splashImRestLSB	Base image for restore 1BA0 is small alien with upside down Y
20CD	splashImRestMSB
20CE	twoPlayers	1 for yes, 0 means 1 player
20CF	aShotReloadRate	Based on the MSB of the player's score ... how fast the aliens reload their shots
20D0		; This is where the alien-sprite-carying-the-Y ...
20D1		; ... lives in ROM
20D2
20D3
20D4
20D5
20D6
20D7
20D8
20D9
20DA
20DB
20DC
20DD
20DE
20DF
20E0
20E1
20E2
20E3
20E4
20E5	player1Ex	Extra ship has been awarded = 0
20E6	player2Ex	Extra ship has been awarded = 0
20E7	player1Alive	1 if player is alive, 0 if dead (after last man)
20E8	player2Alive	1 if player is alive, 0 if dead (after last man)
20E9	suspendPlay	1=game things are moving, 0=game things are suspended
20EA	coinSwitch	1=switch down, 0=switch up (used to debounce coin switch)
20EB	numCoins	number of coin credits in BCD format (99 max)
20EC	splashAnimate	0 for animation during splash and 1 for not. This alternates after every cycle.
20ED	demoCmdPtrLSB	pointer to demo commands LSB 1663
20EE	demoCmdPtrMSB	pointer to demo commands MSB
20EF	gameMode	1=game running, 0=demo or splash screens
20F0
20F1	adjustScore	Set to 1 if score needs adjusting
20F2	scoreDeltaLSB	Score adjustment (LSB)
20F3	scoreDeltaMSB	Score adjustment (MSB)
20F4	HiScorL	Hi-score descriptor ... value LSB
20F5	HiScorM	Hi-score descriptor ... value MSB
20F6	HiScorLoL	Hi-score descriptor ... location LSB
20F7	HiScorLoM	Hi-score descriptor ... location MSB
20F8	P1ScorL	Hi-score descriptor ... value LSB
20F9	P1ScorM	Hi-score descriptor ... value MSB
20FA	P1ScorLoL	Hi-score descriptor ... location LSB
20FB	P1ScorLoM	Hi-score descriptor ... location MSB
20FC	P2ScorL	Hi-score descriptor ... value LSB
20FD	P2ScorM	Hi-score descriptor ... value MSB
20FE	P2ScorLoL	Hi-score descriptor ... location LSB
20FF	P2ScorLoM	Hi-score descriptor ... location MSB

Player 1 specific data

2100:2136		Player 1 alien ship indicators (0=dead) 11*5 = 55
2137:2141		Unused 11 bytes (room for another row of aliens?)
2142:21F1		Player 1 shields remembered between rounds 44 bytes * 4 shields ($B0 bytes)
21F2:21FA		Unused 9 bytes
21FB	p1RefAlienDX	Player 1 reference-alien delta X
21FC	p1RefAlienY	Player 1 reference-alien Y coordinate
21FD	p1RefAlienX	Player 1 reference-alien X coordiante
21FE	p1RackCnt	Player 1 rack-count (starts at 0 but get incremented to 1-8)
21FF	p1ShipsRem	Ships remaining after current dies

Player 2 specific data

2200:2236		Player 2 alien ship indicators (0=dead) 11*5 = 55
2237:2241		Unused 11 bytes (room for another row of aliens?)
2242:22F1		Player 2 shields remembered between rounds 44 bytes * 4 shields ($B0 bytes)
22F2:22FA		Unused 9 bytes
22FB	p2RefAlienDX	Player 2 reference-alien delta X
22FC	p2RefAlienYr	Player 2 reference-alien Y coordinate
22FD	p2RefAlienXr	Player 2 reference-alien X coordinate
22FE	p2RackCnt	Player 2 rack-count (starts at 0 but get incremented to 1-8)
22FF	p2ShipsRem	Ships remaining after current dies

Stack

2300:23DD		Unused (stack space)
23DE:23FF		In the emulator the stack consumes this area (roughly 16 levels)