The Little Mermaid/ROM map

The following article is a ROM map for The Little Mermaid.

Page 06 (0x8000-0xBFFF)

Address in ROM: 0x18010-0x1C00F

Function 0x8000

Runs the action scripts of every object slot from lowest to highest and checks for collisions each frame.

06:8000:A5 B1     LDA $00B1   ; Check Ariel's health
06:8002:F0 3A     BEQ $803E   ; Exit if zero 
06:8004:A2 01     LDX #$01
06:8006:86 EF     STX $00EF   ; Current slot #
06:8008:A9 06     LDA #$06
06:800A:85 99     STA $0099   ; ?

06:800C:BD 00 03  LDA $0300,X 
06:800F:10 25     BPL $8036   ; Skip ahead if slot is empty
06:8011:BD A0 04  LDA $04A0,X ; Check if object is stunned
06:8014:D0 16     BNE $802C   ;  If so, skip action script

06:8016:BC 10 03  LDY $0310,X
06:8019:B9 20 85  LDA $8520,Y ; Obtain address of action script
06:801C:85 00     STA $0000   ;  from jump table
06:801E:B9 80 85  LDA $8580,Y ; 
06:8021:85 01     STA $0001
06:8023:A9 80     LDA #$80    ; Set return address
06:8025:48        PHA
06:8026:A9 2B     LDA #$2B
06:8028:48        PHA
06:8029:6C 00 00  JMP ($0000) ; Run object's action script

06:802C:E0 03     CPX #$03    ; Check if object is a bubble
06:802E:90 06     BCC $8036   ; Jump ahead if so
06:8030:20 3F 80  JSR $803F   ; Test collision with Ariel
06:8033:20 FF 80  JSR $80FF   ; Test collision with bubble

06:8036:E6 EF     INC $00EF   ; Tick up to next object slot
06:8038:A6 EF     LDX $00EF   
06:803A:E0 10     CPX #$10    ; Loop until all slots examined
06:803C:D0 CA     BNE $8008
06:803E:60        RTS -----------------------------------------

Function 0x803F

Checks if Ariel is colliding with object X and performs appropriate action if so.

06:803F:20 F2 FA  JSR $FAF2   ; Check if Ariel is colliding with object X
06:8042:B0 79     BCS $80BD   ; Exit if not

06:8044:BD C0 03  LDA $03C0,X ; Check if item has damage flag
06:8047:29 40     AND #$40    ;
06:8049:F0 73     BEQ $80BE   ; Skip to next section if not

ARIEL COLLIDES WITH DAMAGING OBJECT

06:804B:A5 3F     LDA $003F
06:804D:05 2F     ORA $002F
06:804F:D0 6C     BNE $80BD   ; Exit unless both are zero

06:8051:A5 BB     LDA $00BB
06:8053:D0 68     BNE $80BD   ; Exit if not zero

06:8055:A9 07     LDA #$07
06:8057:C5 30     CMP $0030
06:8059:F0 62     BEQ $80BD   ; Exit if Ariel is already in damage animation

ARIEL TAKES DAMAGE

06:805B:C6 B1     DEC $00B1   ; Tick down health
06:805D:D0 05     BNE $8064   ; Jump ahead if not zero

06:805F:A0 00     LDY #$00
06:8061:8C 80 04  STY $0480   ; Clear Parameter 3

06:8064:A4 30     LDY $0030
06:8066:8C 60 04  STY $0460   ; Preserve Ariel's previous action phase
06:8069:85 30     STA $0030   ; Set damage animation
06:806B:A9 1D     LDA #$1D
06:806D:20 A0 FC  JSR $FCA0
06:8070:C0 04     CPY #$04
06:8072:F0 27     BEQ $809B
06:8074:C0 05     CPY #$05
06:8076:F0 23     BEQ $809B
06:8078:A9 08     LDA #$08
06:807A:8D 30 04  STA $0430
06:807D:A9 00     LDA #$00
06:807F:8D 18 04  STA $0418
06:8082:8D 48 04  STA $0448
06:8085:8D 60 04  STA $0460
06:8088:C0 02     CPY #$02
06:808A:D0 0F     BNE $809B
06:808C:8C 60 04  STY $0460
06:808F:AC B0 03  LDY $03B0
06:8092:30 29     BMI $80BD
06:8094:8D A0 03  STA $03A0
06:8097:8D B0 03  STA $03B0
06:809A:60        RTS -----------------------------------------
06:809B:A9 00     LDA #$00
06:809D:85 3A     STA $003A
06:809F:8D 80 03  STA $0380
06:80A2:A9 01     LDA #$01
06:80A4:8D 90 03  STA $0390
06:80A7:A9 10     LDA #$10
06:80A9:8D 70 04  STA $0470
06:80AC:A9 01     LDA #$01
06:80AE:8D D0 03  STA $03D0
06:80B1:AD 00 04  LDA $0400
06:80B4:29 40     AND #$40
06:80B6:F0 21     BEQ $80D9
06:80B8:A9 02     LDA #$02
06:80BA:8D D0 03  STA $03D0
06:80BD:60        RTS -----------------------------------------

ARIEL COLLIDES WITH CARRYABLE OBJECT

06:80BE:A5 3A     LDA $003A
06:80C0:05 30     ORA $0030
06:80C2:D0 15     BNE $80D9
06:80C4:BD 10 03  LDA $0310,X 
06:80C7:C9 30     CMP #$30    ; Check if object is shell
06:80C9:F0 0C     BEQ $80D7   ; If so, set as carried object
06:80CB:C9 02     CMP #$02    ; Check if object is bubble
06:80CD:D0 0A     BNE $80D9   ; If not, exit

06:80CF:86 3A     STX $003A   ; Set bubble as carried object
06:80D1:A9 00     LDA #$00
06:80D3:9D 80 04  STA $0480,X ; Set bubble's parameter 3 as zero
06:80D6:60        RTS -----------------------------------------
06:80D7:86 3A     STX $003A
06:80D9:60        RTS -----------------------------------------

Data Table 0x8520

Jump table for object action functions

Action #	Address	Description
0x00	0x85E0	Blank (used for Ariel)
0x01	0x85E1	Thrown objects
0x02	0x888F	Enemy in bubble
0x03	0x8A14	Fish, Type 1
0x04	0x867A
0x05	0x8A65	Seahorse
0x06	0x8A9B
0x07	0x8BDC
0x08	0x8BE2	Spitting fish
0x09	0x8CA3
0x0A	0x8CBA	Octopus
0x0B	0x8DC5	Octopus' rock
0x0C	0x9BA7	Stage 1 Boss (Shark): Sending Fish
0x0D	0x9CC4	Stage 1 Boss (Shark): Dashing
0x0E	0x9D37	Fish (Stage 1 Boss fight)
0x0F	0xA3B3	Cannon (Stage 4 Boss fight)
0x10	0x8F89
0x11	0x8FC7
0x12	0x9046	Fish, Type 2
0x13	0xAF50
0x14	0xAF50
0x15	0x908F	Crab (Stage 2 Boss fight)
0x16	0x91D9	Tiny fish (following Ariel)
0x17	0x913F
0x18	0x913F
0x19	0x913F
0x1A	0x913F
0x1B	0x913F
0x1C	0x92A9	Sleeper fish
0x1D	0x9229	Tiny fish (default action)
0x1E	0x9367
0x1F	0x9274	Tiny fish (quad pattern)
0x20	0x940C	Shrimp
0x21	0x9460	Poor Unfortunate Soul (Stage 5)
0x22	0x9461	Starfish spawner
0x23	0x954F	Starfish (shooting upward)
0x24	0x9604	Frozen fish
0x25	0x908F	Helmet crab
0x26	0x869D	Crab (walking)
0x27	0x86C1	Crab (ducking/falling)
0x28	0x86F1
0x29	0x8715
0x2A	0x9D3A	Stage 3 Boss (Walrus): Default
0x2B	0x9EC7	Stage 3 Boss (Walrus): Dropping item
0x2C	0x9F60	Stage 2 Boss (Eel) #1
0x2D	0x9F60	Stage 2 Boss (Eel) #2
0x2E	0xA1CF	Stage 4 Boss (Seahorse)
0x2F	0xA324	Fish (Stage 4 Boss fight)
0x30	0x986D	Carried item
0x31	0x97D5
0x32	0x97D5
0x33	0x97D5
0x34	0x9A36
0x35	0x9A36
0x36	0x9A11
0x37	0x87EE
0x38	0x998B	Red Orb
0x39	0x998B	Green Orb
0x3A	0x9A69
0x3B	0x9A69
0x3C	0x8DCB	Volcano
0x3D	0x878B
0x3E	0xA63D
0x3F	0x888E
0x40	0x888E	Water splash
0x41	0x8EE3
0x42	0x877D	End-of-stage jar
0x43	0x8DCB	Upside-down volcano
0x44	0x8E8C	Volcanic rock
0x45	0x9654
0x46	0x96E1
0x47	0xA3D9	Stage 5 Boss (Ursula 1)
0x48	0xA54F	Enemy Spawn 1 (Stage 5 Boss fight)
0x49	0xA5F0	Enemy Spawn 2 (Stage 5 Boss fight)
0x4A	0xA5F6	Orb (Stage 5 Boss fight)
0x4B	0x8F6E
0x4C	0x876B
0x4D	0x9AA1	Hidden item (?)
0x4E	0x9AA1	Hidden item (?)
0x4F	0x9AA1	Hidden item (?)
0x50	0x9AA1	Hidden item (?)
0x51	0x9AA1	Hidden item (?)
0x52	0x9B0A	Flounder
0x53	0x9B0A
0x54	0x9B0A
0x55	0x9B0A
0x56	0x9B0A
0x57	0xA692	Stage 6 Boss (Ursula 2)
0x58	0xA83E	Fish (Stage 6 Boss fight)
0x59	0xA879
0x5A	0xA0D8	Crab spawner (Stage 2 Boss fight)
0x5B	0xA324
0x5C	0x8821
0x5D	0x8872
0x5E	0x887F
0x5F	0x888E	Urchin (Stage 4 Boss fight)

Function 0x85E0

Action script 0x00 (Ariel)

06:85E0:60        RTS -----------------------------------------

Page 07 (0xC000-0xFFFF)

Location in ROM: 0x1C010-0x2000F

Function 0xC250

Advance random number generator each frame

07:C250:A2 00     LDX #$00
07:C252:A0 04     LDY #$04
07:C254:B5 E4     LDA $E4,X
07:C256:29 02     AND #$02
07:C258:85 00     STA $0000
07:C25A:B5 E5     LDA $E5,X
07:C25C:29 02     AND #$02
07:C25E:45 00     EOR $0000 ; If 0x02 bits of 0xE4 and 0xE5
07:C260:18        CLC       ;  are the same, highest bit of
07:C261:F0 01     BEQ $C264 ;   new RNG value will be zero.
07:C263:38        SEC       ;    Otherwise, it will be one.
07:C264:76 E4     ROR $E4,X ;     Then shift all previous bits right.
07:C266:E8        INX
07:C267:88        DEY
07:C268:D0 FA     BNE $C264
07:C26A:60        RTS -----------------------------------------