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The Little Mermaid/ROM map
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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 $8520,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 ; Run action if object is colliding with Ariel 06:8033:20 FF 80 JSR $80FF ; Run action if object is colliding with a 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 -----------------------------------------
Data Table 0x8520
Jump table for object action functions
| Action # | Address | Description |
| 0x00 | 0x85E0 | Blank |
| 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) |
Page 07 (0xC000-0xFFFF)
Location in ROM: 0x1C010-0x2000F
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 -----------------------------------------