# Donkey Kong 3 (JUE) — Game Flow from Power-On

This document describes the execution flow of the disassembled Donkey Kong 3 (JUE) PRG bank (16 KB, mapped at CPU **$C000–$FFFF**), starting from power-on. It is based entirely on `copilot/prg.s` in the repository. The rebuild is verified **byte-identical** to the original ROM (whole-ROM compare).

---

## 1. Reset Vector — CPU $FFFC → $C000

**`ResetHandler`** ($C000–$C08C)

On power-on or reset, the 6502 jumps to `$C000`. The reset routine performs cold-start hardware initialization and enters the main loop dispatcher:

| Step | Code | Purpose |
|------|------|---------|
| 1–2 | `cld` / `sei` | Clear decimal mode, disable interrupts |
| 3–6 | Poll `$2002` twice (`bpl *-3`) | Wait for two full VBlank periods (PPU warm-up) |
| 7–8 | `ldx #$00` / `txs` | Initialize stack pointer to $01FF |
| 9–11 | `ldx FrameCounter` / `ldy #$07` / `sty WorkPointerOrValueHi` | FrameCounter ($27) → X; Y=$07 → WorkPointerOrValueHi ($02) |
| 12 | `ldy #$00` / `sty WorkPointerOrValueLo` | Y=$00 → WorkPointerOrValueLo ($01) |
| 13–21 | Clear RAM $0000–$07FF | 8 pages via (WorkPointerOrValueLo),Y; dec WorkPointerOrValueHi loops pages |
| 22 | `txa` / `bne FinishResetInitialization` | If FrameCounter was non-zero, skip full clear |
| 23 | `ldx #$5F` | Alternate init value for warm start |
| 24 | `stx Ram_06F0` | Store signature at $06F0 |
| 25 | `jsr ClearNametable2400WithTileFF` ($C1A2) | Fill nametable $2400 with $FF |
| 26 | `jsr HideAllOamSprites` ($C1EA) | Hide all 64 sprites (Y=$F4) |
| 27 | `sty $2005` ×2 | PPUSCROLL = 0,0 |
| 28 | `iny` / `sty GameMode` | Y=$01 → GameMode ($3A) = title mode |
| 29 | `lda #$0F` / `sta $4015` | Enable all APU channels |
| 30 | `lda #$90` / `sta $2000` / `sta PPUCTRL` | PPUCTRL: NMI on, sprite $0000, BG $1000, 8×8 sprites |

**Warm-start signature:** `FrameCounter` ($27) and `Ram_06F0` survive the RAM clear. If `FrameCounter` ≠ 0 on reset, the game skips the full 2 KB clear and uses X=$5F.

---

## 2. Main Loop Dispatcher — `SelectMainLoopAudioPath` ($C047)

After reset initialization, control flows to **`SelectMainLoopAudioPath`** ($C047), the main loop dispatcher:

```
SelectMainLoopAudioPath ($C047):
    lda GameMode           ; $3A
    beq RunAudioOnlyFrame  ; 0 = title/attract: audio only
    lda PlayerModeOrScreenPhase  ; $3C
    cmp #$01
    bne RunMainGameFrame   ; !=1: gameplay frame
RunAudioOnlyFrame ($C051):
    jsr UpdateAudioEngine  ; $F820 (banked audio)
RunMainGameFrame ($C054):
    jsr DecrementGameplayTimers      ; $C3B0
    jsr UpdateTitleAndGameplayState  ; $C0BA
    jsr RequestNmiProcessing         ; $C05F
    beq SelectMainLoopAudioPath      ; loop until NMI signals frame ready
```

**`RequestNmiProcessing`** ($C05F): Sets `NmiFrameDelay` ($32) = 1, signaling NMI to do work.

**`WaitForFrameReady`** ($C063): Called after `RequestNmiProcessing` returns (via `beq` loop):
```
    jsr AdvancePseudoRandomState  ; $C42F (PRNG)
    lda FrameReadyFlag            ; $30
    beq WaitForFrameReady         ; wait for NMI to set it
    inc FrameCounter              ; $27
    dec NmiFrameDelay             ; $32
    lda #$00 / sta FrameReadyFlag ; clear for next frame
    rts
```

---

## 3. Interrupt Vectors — CPU $FFFA–$FFFF

Located at PRG offset $3FFA (end of 16 KB bank):

| Vector | Address | Handler |
|--------|---------|---------|
| NMI | $C08D | `NmiHandler` |
| RESET | $C000 | `ResetHandler` |
| IRQ | $C000 | `ResetHandler` (reused — unused placeholder) |

---

## 4. NMI / Frame Heartbeat — `NmiHandler` ($C08D)

The NMI fires once per frame and performs PPU updates, controller reading, and signals frame completion:

```
NmiHandler ($C08D):
    pha / txa / pha / tya / pha           ; preserve A,X,Y
    lda #$00 / sta $2003                   ; OAMADDR = 0
    lda #$02 / sta $4014                   ; OAM DMA from $0200
    lda NmiFrameDelay  ($32)
    beq CompleteNmiAndRestoreRegisters    ; if 0: minimal NMI (title screen)
    jsr RunQueuedPpuTransfer       ; $C1F5 — process queued PPU writes
    jsr UpdatePpuTransferQueue     ; $C370 — update queue pointers
    jsr ResetPpuScroll             ; $C424 — reset scroll registers
    jsr UpdateControllerState_C328 ; $C328 — read controllers
    ldy #$01 / sty FrameReadyFlag  ; $30 = 1 (signal main loop)
CompleteNmiAndRestoreRegisters ($C0B0):
    lda #$01 / sta NmiWorkMode     ; $33 = 1 (signal NMI work done)
    pla / tay / pla / tax / pla / rti
```

**Two NMI modes:**
- **NmiFrameDelay = 0** (title/attract): Only OAM DMA, no game logic.
- **NmiFrameDelay = 1** (gameplay): Full PPU queue flush, controller read, scroll reset, sets `FrameReadyFlag`.

---

## 5. Title / Gameplay State Machine — `UpdateTitleAndGameplayState` ($C0BA)

```
UpdateTitleAndGameplayState ($C0BA):
    lda GameMode ($3A)
    bne HandleStartAndPlayerSelection  ; !=0: player select
    lda ControllerButtons ($1A)
    and #$10        ; START button
    beq ClearScreenSubstate
    ldy ScreenSubstate ($36)
    bne DispatchCurrentScreenState
    iny / sty ScreenSubstate           ; first START press: advance substate
    ldy CurrentScreenState ($3B)
    cpy #$0D / beq StartGameplayCountdown
    cpy #$06 / bne DispatchCurrentScreenState
    ldx #$07
    jsr CopyPlayerStateDefaults        ; init player timers
    ; ... sets CurrentScreenState=$0D, PPUMASK=$0E, PPUCTRL=$81
    jsr ClearAudioRequestFlags
    beq SelectAlternateAudioMode       ; audio request $02
ClearScreenSubstate ($C104):
    lda #$00 / sta ScreenSubstate
DispatchCurrentScreenState ($C108):
    lda CurrentScreenState ($3B)
    jsr DispatchScreenStateHandler ($C3C6)
ScreenStateHandlerTable ($C10D): 14 entries (2 bytes each)
    .word Handler_$C663, $C6AB, $C6D4, $C6EB, $C755, $C7C2, $C7DD, $C8C6, $C8C8
    .word $C93E, $C94B, $C998, $C9AF, $C9C7, $C07C
```

**GameMode = 0 (Title):**
- Waits for START press (`ControllerButtons & $10`)
- `ScreenSubstate` tracks first press vs. subsequent
- `CurrentScreenState` selects which title screen handler (14 states)
- State $0D = start gameplay countdown (sets `GameplayTimer`=$14, `CurrentScreenState`=$0C)
- State $06 = player select (copies defaults, sets up audio)

**GameMode ≠ 0 (Player Select / Gameplay):**
- `HandleStartAndPlayerSelection` ($C129): reads START/SELECT, sets `GameMode`/`CurrentScreenState`
- `DispatchGameModePhase` ($C142): dispatches on `PlayerModeOrScreenPhase` ($3C)
- Screen state handlers at $C18D via second table ($C192): 15 entries for gameplay screens

---

## 6. Key Subsystems

### 6.1 PPU Transfer Queue — `$C1F5` / `$C370`
- `QueuedPpuTransferCount` ($34) tracks pending transfers
- Source pages from `QueuedPpuTransferSourcePageTable` ($C20A)
- Destination offsets from `QueuedPpuTransferDestinationOffsetTable` ($C206)
- `RunQueuedPpuTransfer` processes one per NMI; `UpdatePpuTransferQueue` refills

### 6.2 Controller Input — `UpdateControllerState_C328` ($C328)
- Reads `$4016` (strobe) + shift register
- Stores buttons in `ControllerButtons` ($1A), pressed edges in `ControllerPressedOrScriptInput` ($1B)
- Second controller in `SecondControllerPressed` ($1D)

### 6.3 Audio Engine — `UpdateAudioEngine` ($F820)
- Banked at $F800+ (called via trampoline)
- 4-channel sequencer: `AudioChannelStateArray` ($F0–$F3), sequence pointers ($F9/$FA)
- Note/period tables at $F970, $FAB3
- Sound effects via `StartSoundEffect` ($FAE6)

### 6.4 Screen State Handlers — `DispatchScreenStateHandler` ($C3C6)
- Title: 14 handlers (attract, copyright, high score, player select, etc.)
- Gameplay: 15 handlers (level intro, gameplay, bonus, intermission, game over, etc.)
- Each handler manages its own objects, timers, and transitions

### 6.5 Object / Actor System
- `ObjectState` ($0300+): 16+ slots, fields for type, position, timer, animation
- `ActorFlags` ($84), `ActorX` ($85), `ActorY` ($86), `ActorStateArray` ($70+)
- Collision detection via `ScreenCollisionState` ($44), `CollisionMask` ($BF)

### 6.6 PRNG — `AdvancePseudoRandomState` ($C42F)
- Called once per frame in `WaitForFrameReady`
- Used for enemy spawn patterns, bonus timing

---

## 7. Memory Map (Key Zero Page / RAM)

| Address | Name (from `locals.inc`) | Use |
|---------|--------------------------|-----|
| $00 | `ControllerSerialBits` | Controller shift register bits |
| $01–$02 | `WorkPointerOrValueLo/Hi` | General 16-bit pointer (RAM clear, PPU) |
| $03 | `SourceRecordPointerLo` | PPU transfer source pointer |
| $1A | `ControllerButtons` | Current button state |
| $1B | `ControllerPressedOrScriptInput` | Edge-detected presses |
| $20–$27 | `GameplayTimerArray` | 8 gameplay timers |
| $21 | `GameplayTimer` | Main gameplay countdown |
| $25 | `ScreenSelectionPositionOrDelay` | Menu cursor / delay |
| $27 | `FrameCounter` | Global frame counter |
| $28–$2F | `SavedGameplayTimerArray` | Backup timers |
| $30 | `FrameReadyFlag` | Set by NMI, cleared by main loop |
| $31 | `CollisionResultFlag` | Collision detection result |
| $32 | `NmiFrameDelay` | NMI work trigger (0=skip, 1=do work) |
| $33 | `NmiWorkMode` | NMI completion signal |
| $34 | `QueuedPpuTransferCount` | Pending PPU transfers |
| $36 | `ScreenSubstate` | Title screen sub-state |
| $37 | `StartInputLatch` | START button latch |
| $38 | `SelectedPlayerAndScreenFlags` | Player/screen selection |
| $3A | `GameMode` | 0=title, 1=player select, 2+=gameplay |
| $3B | `CurrentScreenState` | Active screen handler index |
| $3C | `PlayerModeOrScreenPhase` | Player mode / screen phase |
| $3D | `ReturnScreenState` | Return destination |
| $40–$41 | `SelectedPlayerIndex`, `SelectedScreenVariant` | Menu selections |
| $50+ | `GameplayObjectState`… | Object pool ($0300+) fields mirrored in ZP |
| $F0–$FF | Audio engine ZP | Channel state, sequences, envelopes |
| $0200 | `OamBuffer` | OAM DMA source (64 sprites) |
| $0300+ | `ObjectState`… | Main object/actor array |
| $06F0 | `Ram_06F0` | Warm-start signature |
| $0700+ | Extended RAM | Actor timers, collision, audio buffers |

---

## 8. Data Tables

| Table | Address | Description |
|-------|---------|-------------|
| `ScreenStateHandlerTable` | $C10D | 14 title screen handlers |
| `ScreenStateHandlerTable` (gameplay) | $C192 | 15 gameplay screen handlers |
| `QueuedPpuTransferSourcePageTable` | $C20A | PPU transfer source banks |
| `QueuedPpuTransferDestinationOffsetTable` | $C206 | PPU dest offsets (alias at $C206) |
| `PlayerStartXTable` | $8C | Player start X positions |
| `PlayerStartYTable` | $8D | Player start Y positions |
| `AudioPitchOrPeriodTable_F970` | $F970 | Note→period lookup |
| `AudioSequenceIndexTable_FAB3` | $FAB3 | Sequence index table |
| `IndexedGameplayLookupTable_FFC3` | $FFC3 | Gameplay bitmask/offset table |
| `GameplayConstantTable_FF4C` | $FF4C | Level/gameplay constants |

---

## 9. Summary of Execution Flow

```
Power On / Reset
    │
    ▼
ResetHandler ($C000) ────► cld/sei, wait 2 VBlanks, init stack
    │
    ├─ Clear $0000–$07FF (8 pages via WorkPointerOrValueLo/Hi)
    │     └─ Warm-start: if FrameCounter≠0, skip clear, X=$5F
    │
    ├─ stx Ram_06F0 (signature)
    ├─ jsr ClearNametable2400WithTileFF ($C1A2)
    ├─ jsr HideAllOamSprites ($C1EA)
    ├─ PPUSCROLL = 0,0
    ├─ GameMode = 1 (title)
    ├─ APU enable ($4015=$0F)
    ├─ PPUCTRL = $90 (NMI on)
    │
    ▼
SelectMainLoopAudioPath ($C047) ─────────────────────────────┐
    │                                                         │
    ├─ GameMode=0: jsr UpdateAudioEngine ($F820), loop       │
    │                                                         │
    └─ GameMode≠0:                                            │
         jsr DecrementGameplayTimers ($C3B0)                  │
         jsr UpdateTitleAndGameplayState ($C0BA)              │
         jsr RequestNmiProcessing ($C05F)  → NmiFrameDelay=1  │
         jsr WaitForFrameReady ($C063)  ◄── wait for NMI ────┘
              │
              ├─ jsr AdvancePseudoRandomState ($C42F)
              ├─ wait for FrameReadyFlag ($30)
              ├─ inc FrameCounter ($27), dec NmiFrameDelay
              └─ clear FrameReadyFlag, rts
         beq SelectMainLoopAudioPath (loop)

NMI ($C08D) [once per frame]
    ├─ Preserve A,X,Y
    ├─ OAM DMA ($4014 from $0200)
    ├─ lda NmiFrameDelay ($32)
    │     ├─ 0: CompleteNmiAndRestoreRegisters (title)
    │     └─ 1: RunQueuedPpuTransfer ($C1F5)
    │           UpdatePpuTransferQueue ($C370)
    │           ResetPpuScroll ($C424)
    │           UpdateControllerState_C328 ($C328)
    │           ldy #$01 / sty FrameReadyFlag ($30)
    ├─ lda #$01 / sta NmiWorkMode ($33)
    └─ Restore A,X,Y, rti

Title Screen ($C0BA, GameMode=0)
    ├─ Wait for START (ControllerButtons & $10)
    ├─ ScreenSubstate tracks first/second press
    ├─ CurrentScreenState selects handler (14 states at $C10D)
    ├─ State $0D: StartGameplayCountdown → GameplayTimer=$14, State=$0C
    └─ State $06: CopyPlayerStateDefaults → init timers, audio

Player Select / Gameplay ($C129, GameMode≠0)
    ├─ HandleStartAndPlayerSelection: START/SELECT → GameMode, CurrentScreenState
    ├─ DispatchGameModePhase ($C142): PlayerModeOrScreenPhase ($3C) → handler
    ├─ DispatchScreenState ($C18D): 15 gameplay handlers ($C192 table)
    └─ Gameplay loop: objects, collision, scoring, level progression
```

---

## 10. Naming and Residue

**Pass 1/2/3 complete** — `prg.s` has **zero** `Label_` definitions. All routines carry semantic names from Pass 2 (`ResetHandler`, `NmiHandler`, `UpdateTitleAndGameplayState`, `DispatchScreenStateHandler`, etc.) and internal branch targets use `<Routine>_Block[_n]` convention.

**Pass 3 symbols applied (2026-10-19):** `_tools/apply_symbols.py` substituted **75 operands** (17 single-name + 58 hardware) from `symbols.inc`/`locals.inc`. Zero residue.

**Overlapping table aliases** (expressed as relocatable symbols):
- `QueuedPpuTransferDestinationOffsetTable = RunQueuedPpuTransfer + $11` ($C206)
- `AudioPitchOrPeriodTable_F970 = Flow_F96F + $01` ($F970)
- `AudioSequenceIndexTable_FAB3 = Flow_FA85 + $2E` ($FAB3)

**Vectors** at PRG $3FFA: `.word NmiHandler, ResetHandler, ResetHandler`

Build verification: `make` in `copilot/` produces `reconstructed.nes`, compared whole-ROM against `rom.nes` (byte-identical).