# Excitebike (JU) — Game Flow from Power-On

This document describes the execution flow of the disassembled Excitebike (JU) 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 `prg.bin`.

---

## 1. ROM and Memory Layout

| Property | Value |
|----------|-------|
| Format | iNES / NES 2.0 |
| PRG | 1 × 16 KB (mapped at CPU $C000–$FFFF) |
| CHR | 1 × 8 KB |
| Mirroring | Vertical |
| PRG bank start | CPU $C000 |
| Source file | `copilot/prg.s` (14,111 lines) |
| Output | `reconstructed_prg.bin` (16,384 bytes) |

### Interrupt vectors (CPU $FFFA–$FFFF)

| Vector | Address | Handler |
|--------|---------|---------|
| NMI | $C23B | `NMI` |
| RESET | $C184 | `Reset` |
| IRQ / BRK | $FFF0 | (`$FFF0` — points into audio data region, unused) |

---

## 2. Build Status

`make` in `copilot/` assembles `prg.s` with `ca65`, links with `ld65`, and verifies against `prg.bin`.

**Verification result: BYTE-IDENTICAL** — `reconstructed_prg.bin` matches `prg.bin` exactly (headers, CHR, and all PRG bytes).

### Build fix applied

The original source contained a `bpl LookupTableEntry_FAF5` branch at CPU $FAB4 (line 9425 of `prg.s`), but the symbol `LookupTableEntry_FAF5` was undefined. The branch target CPU $FAF5 fell in the middle of what the disassembler classified as `lda #$16` (`A9` at $FAF4, `16` at $FAF5). The fix:

- Replaced `bpl LookupTableEntry_FAF5` with `bne *-$1B` at CPU $FAB4, producing the exact original bytes `d0 e3` (the disassembler had mislabeled `bne` as `bpl`).

---

## 3. Reset — CPU $FFFC → $C184

**`Reset`** ($C184)

On power-on or reset:

1. `sei` — disable maskable interrupts.
2. `cld` — clear decimal mode.
3. `lda #$00 / sta PPUCTRL` — disable PPU rendering and NMI.
4. `ldx #$FF / txs` — initialize stack pointer to $FF.
5. **Wait for two VBlanks** — poll `PPUSTATUS` bit 7 twice (`@vblank1`, `@vblank2`). This is the PPU warm-up delay.
6. **Reset magic check** — compare `ResetMagic1` ($05CE) against `$A5` and `ResetMagic2` ($05CF) against `$5A`:
   - If both match → warm start: set Y = $04 (preserve more RAM)
   - If either mismatches → cold start: set Y = $07 (clear more RAM)
7. **Clear RAM** — clear zero page and one page ($0000–$00FF, or $0000–$06FF depending on warm/cold start) using indirect pointer at $00/$01.
8. **Persistent RAM init** — if reset magic was valid, `jsr InitializePersistentRam` ($C278):
   - Copy 31-byte template (`PersistentRecordInitialDataC091` at $C091) to both `PersistentRecord0` ($0580) and `PersistentRecord1` ($05A3)
   - Fill `PersistentBuffer` ($06E0) with $40, set byte at $07AF to $09
   - Set reset magic bytes to $A5/$5A
   - Zero tail bytes of both persistent records
9. `sta APUDMALOAD` — disable DMC.
10. `lda #$06 / sta PPUMASK / sta $18` — enable background + sprites, set $18.
11. `jsr InitializeRuntimePpuState` — fill OAM shadow with $F8, process PPU entries, zero scroll/render fields.
12. `lda #$90 / jsr SetPpuControl` — set PPUCTRL = $90 (NMI enabled, $2000 base).
13. Falls into **`@main`** ($C1A4) — the permanent main loop.

---

## 4. Main Loop — `@main` ($C1A4)

After reset, `Reset` never returns. It enters the infinite main loop:

```
@main ($C1A4):
    jsr ShiftRuntimeState        ; $E8BC — shift runtime bytes $18-$1F right
    lda NmiSyncState             ; $48
    bmi @main                    ; loop if $FF (NMI_SYNC_PENDING)
    beq @main                    ; loop if $00
    ; NmiSyncState == $01 (NMI_SYNC_COMPLETE): proceed
    jsr DisableNmi               ; $C362 — clear NMI bit in PPUCTRL
    lda NmiSyncState
    cmp #NMI_SYNC_COMPLETE       ; $01
    bne @alternate               ; if not complete, receive instead
    jsr TransmitRuntimeDataCode  ; $C6C8 — serial transmit runtime data
    jmp @finish
@alternate:
    jsr ReceiveRuntimeDataCode   ; $C739 — serial receive runtime data
@finish:
    lda #NMI_SYNC_PENDING        ; $FF
    sta NmiSyncState             ; $48 — reset sync flag for next frame
    jsr WaitForVblankEndAndEnableNmi  ; wait for VBlank, set NMI bit
    jmp @main                    ; loop forever
```

The main loop is **NMI-synchronized**: it blocks until `NmiSyncState` ($48) becomes $01 (set by NMI each frame), then performs serial I/O (transmit or receive runtime data), resets the sync flag to $FF, waits for the next VBlank, enables NMI, and repeats.

### `ShiftRuntimeState` ($E8BC)

Shifts runtime state bytes $18–$1F right by one bit. The carry-in is derived from controller bit 1 ($19 bit 2), enabling serial/data shifting tied to controller input.

---

## 5. NMI / Frame Heartbeat — `NMI` ($C23B)

The NMI fires once per frame and is the central real-time driver:

1. **`jsr DisableNmi`** ($C362) — clear bit 7 of `PpuControlShadow` ($10), write to PPUCTRL ($2000). NMI is disabled during the handler to prevent re-entry.
2. **PPU mask**: if $45 == 0, `PPUMASK = PpuMaskShadow | $1A` (enable background + sprites); otherwise skip.
3. **OAM DMA**: write $00 to OAMADDR ($2003), then $02 to OAMDMA ($4014) — copy $0200–$02FF to sprite RAM.
4. **`jsr NmiPpuUpdate`** ($C27E) — process one PPU update command stream (indexed by $45), then reset $0300/$0301/$45 and PPU address to $3F00.
5. **Optional rider/lane update**: if $47 != 0, `jsr UpdateRiderLanePositionStates` and `jsr NmiRuntimeUpdate`.
6. **Runtime counters**: if $03B1 | $03B2 == 0, increment $3F and `jsr UpdateRuntimeCounters`.
7. **`jsr ReadControllers`** ($E8E5) — read both controller ports ($4016/$4017), update `Controller1Held` ($14), `Controller1Pressed` ($15), `Controller2Held` ($16), `Controller2Pressed` ($17).
8. **`jsr UpdateAudio`** ($F8DE) — update all four APU channels (Pulse 1, Pulse 2, Noise, Sound Effect) plus sound effects and DMA.
9. **`jsr DispatchNmiState`** ($C2A9) — dispatch to the current NMI state handler (see below).
10. **Read PPUSTATUS** ($2002) to reset the address latch.
11. **`jsr WaitForVblankEndAndEnableNmi`** — wait for VBlank end, set NMI bit in PPUCTRL.
12. **`rti`** — return from interrupt.

### `NmiPpuUpdate` ($C27E)

Processes a single PPU update command:
- Load pointer from `PpuUpdatePointerLow/High` ($C0F6/$C111 tables) indexed by $45.
- Call `ProcessPpuUpdateStream` ($D3C1) which reads the command stream and writes bytes to $2007.
- Zero out $0300, $0301, and $45.
- Reset PPU address to $3F00 (four writes to $2006).

---

## 6. NMI State Machine — `DispatchNmiState` ($C2A9)

The NMI dispatcher supports two modes based on `NmiDispatchMode` ($40):

### Mode 0 — Direct state dispatch (`DispatchNmiState` @ $C2A9)
If `NmiDispatchMode` == 0:
- Load `NmiState` ($41), double it, use as index into `NmiStateHandlerTable` ($C000–$C01B).
- Jump indirect through zero-page $00/$01 (high byte left in A, stored to $01 by code at $CB36).

**NMI State Handler Table** ($C000–$C01B) — 14 entries for states $00–$0D:

| State | Handler Address | Purpose |
|-------|----------------|---------|
| $00 | `NmiState00` | Menu/title — selection handling |
| $01 | $C395 | Menu horizontal input |
| $02 | `UpdateGameplayRuntime` | Main gameplay update |
| $03 | `DispatchNmiSequenceStep` | Sequence step dispatch |
| $04 | `NmiState04_07` | Shared state $04/$07 handler |
| $05 | `UpdateGameplayRuntime` | Main gameplay update |
| $06 | `DispatchNmiSequenceStep` | Sequence step dispatch |
| $07 | `NmiState04_07` | Shared state $04/$07 handler |
| $08 | $C3BD | Menu vertical input |
| $09 | `UpdateRaceRuntime` | Race gameplay update |
| $0A | `UpdateGameplayRuntime` | Main gameplay update |
| $0B | `DispatchNmiSequenceStep` | Sequence step dispatch |
| $0C | $C43D | Serial handshake state $0C |
| $0D | $C41C | Serial handshake state $0D |

### Mode 1 — Sequence dispatch
If `NmiDispatchMode` != 0:
- Disable PPU rendering.
- Wait until $30 == 0 (not busy).
- Look up sequence pointer from `NmiStateToSequenceIndex` + `NmiSequencePointerLow/High` tables.
- Call the sequence handler at the current `NmiSequenceStep` ($44).
- If step < 0 (after increment/decrement), go to `InitializeRuntimePpuState`.

**NMI Sequence Pointers** ($C03C–$C07F) — 9 sequences:

| Index | Address | Handlers |
|-------|---------|----------|
| 0 | $C03C | $C2BD, $C514 |
| 1 | $C040 | `SetSerialUpdateSelector06`, $C3F3, $C514 |
| 2 | $C046 | `SerialUpdateSelectorCluster`, `ClearRuntimeZeroPage`, `InitializeCourseRuntime`, `SetSerialUpdateSelector17`, `SetSerialUpdateSelector0E`, `InitializeGameRuntime` |
| 3 | $C052 | `SetSerialUpdateSelector06`, `SetSerialUpdateSelector04`, `SelectNmiUpdateForState`, `CallC92CWithZero`, `CompareAndUpdateRecord` |
| 4 | $C05C | `SetSerialUpdateSelector06`, `SetSerialUpdateSelector09`, `Prepare0300Structure`, $C514 |
| 5 | $C064 | $C551, `ClearRuntimeZeroPage`, `InitializeCourseRuntime`, `SetSerialUpdateSelector17`, $C562, $C5C2 |
| 6 | $C070 | `SetSerialUpdateSelector06`, $C575, $C514 |
| 7 | $C07A | `ClearRuntimeZeroPage`, `RestoreWorkingRecordFields`, $C518 |
| 8 | $C076 | $C582, $C518 |

---

## 7. Key NMI State Handlers

### `NmiState00` ($C346)
Title/menu state:
- If $31 == 0: initialize $43, $4A = 0, $03F7 = $0F, `NmiState` = 2 (gameplay).
- Else: `jsr UpdateMenuSelection` with count 3; if no input change, set sprite 0 Y from `MenuInitialYTable`; otherwise handle selection done and advance state.

### `NmiState01` ($C397)
Menu horizontal input:
- Filter controller buttons, `jsr UpdateMenuSelection` (count 5).
- If input changed, move horizontal marker; else store `MenuSelectionIndex` to $43 and advance to next NMI state, entering sequence mode.

### `NmiState08` ($C3BD)
Menu vertical input:
- `jsr UpdateMenuSelection` (count 6).
- Based on selection: advance state, jump to sequence entry, or handle choice 5 (reset runtime, preserve selection).

### `NmiState04_07` ($C465)
Shared handler for states $04/$07:
- If $34 == 0: advance NMI state and enter sequence mode; clamp state to max 7.
- Else: `jsr $C92C`, check bit 4 of $3F, either return or `jmp StateHelperCAFA`.

### `NmiState0C` / `NmiState0D` ($C41C / $C43D)
Serial handshake states:
- State $0C: if sequence step == 0, set sync complete; else if sync pending, copy record tails and continue handshake.
- State $0D: similar handshake with persistent record synchronization.

---

## 8. Gameplay Runtime

### `UpdateGameplayRuntime` ($C99A)
Called from NMI states $02, $05, $0A:
- `jsr $CA08` (update four rider slots).
- If $03B1 | $03B2 != 0: jump to PPU update tail.
- Otherwise: `jsr $D918`, `jsr $DD8D`, `jsr UpdateAllRiderScriptStateE733`, `jsr $E927`.
- If $52 != 0: `jsr StateHelperCA9B`.
- If $03F2 != 0: optional updates ($DA9F, $DBC4, $DEBB).
- `jsr UpdateGlobalCounters`, `jsr BuildGlobalDisplayEAC5`, `jsr CompareRuntimeSlots`, `jsr CheckSlotState`.
- `jsr UpdateTriggeredSlots`, `jsr UpdateFourSlots`, `jsr UpdateFourSlotPositionsE96C`, `jsr UpdateDisplayState`, `jsr UpdateFourSlot8CE836`, `jsr BuildFourSlotSpritesEA44`, `jsr ProcessCourseDefinition`, `jsr UpdateSlotPairState`, `jsr UpdateGlobalFC`, `jsr UpdateGlobalDisplay`, `jsr ProcessRuntimeEntries`, `jsr PrepareSortedSlots`, `jsr UpdateRuntimeSlots`, `jsr UpdateRiderCourseStateE70B`, `jsr UpdatePpuState`, `jsr ProcessRiderSlots`, `jmp ProcessSortedSlots`.

### `UpdateRaceRuntime` ($C8D6)
Race-specific update:
- `jsr $CA1B`, `jsr UpdateGlobalRuntimeEB17`, `jsr UpdateAccumulator`, `jsr UpdateGlobalPosition`, `jsr UpdateRiderCourseStateE70B`, `jsr ProcessCourseDefinition`, `jsr $F68D`, `jsr $F755`, `jsr $C229`, then `jmp UpdatePpuState`.

---

## 9. Audio System

**`UpdateAudio`** ($F8DE) — called from NMI:
1. `APUFRAMECNT` ($4017) = $C0.
2. `jsr UpdatePulse1Channel`, `jsr UpdatePulse2Channel`, `jsr UpdateNoiseChannel`, `jsr UpdateSoundEffect`.
3. Zero out $FF, $FE, $FD, $FB.
4. If $07FF != 0: `sta APUDMALOAD` ($4011), decrement $07FF.

Audio channel configuration uses tables at $FF00+ (period tables) and $FF3E+ (mask tables). Audio sequence/data lives at $FF64–$FFE9.

---

## 10. Serial I/O System

Excitebike uses the controller port $4016 for serial communication (likely the Famicom 3D System or a similar serial peripheral — peripheral identity unknown):

**Transmit path** (`TransmitRuntimeDataCode` at $C6C8):
- Configure a 64-byte block from $0060, transmit it.
- Configure a 20-byte block from $05E0 (working buffer), transmit it.
- Uses pulse-length encoding: long pulse = 1, short pulse = 0, MSB first.
- Preamble: short pulses, then long pulses, then short pulses, then one long pulse.

**Receive path** (`ReceiveRuntimeDataCode` at $C739):
- Disable NMI, configure receive blocks, receive data.
- Validate 5-byte header against `ReceiveHeaderTableBaseC17F` ($C17F: $E8).
- If valid: set state variables ($A0–$A3), set $45 = $1A, wait for NMI update.
- Receive second block (working buffer), validate checksum, return success/failure.

Serial helpers: `TransmitByte` ($C66E), `ReceiveByte` ($C6A5), `TransmitPulseShort` ($C64E), `TransmitPulseLong` ($C653), `ReceiveSerialBit` ($C691), `SerialWaitPulse` ($C639).

---

## 11. Zero-Page / RAM Symbols

| Address | Symbol | Use |
|---------|--------|-----|
| $00–$01 | TMP00/TMP01 | General scratch / indirect pointer |
| $08–$09 | SerialBitCountLo/Hi | Serial bit counter |
| $0E | SerialBlockPtr | Serial block pointer |
| $0D | SerialBlockLength | Serial block length |
| $10 | PpuControlShadow | $2000 mirror |
| $11 | PpuMaskShadow | $2001 mirror |
| $12–$1F | — | Runtime state bytes (shifted each frame by `ShiftRuntimeState`) |
| $14 | Controller1Held | Controller 1 held buttons |
| $15 | Controller1Pressed | Controller 1 newly pressed |
| $16 | Controller2Held | Controller 2 held buttons |
| $17 | Controller2Pressed | Controller 2 newly pressed |
| $31 | — | Menu/timing state |
| $34 | — | Menu selection state |
| $3F | — | Runtime counter |
| $40 | NmiDispatchMode | NMI dispatch mode (0=direct, 1=sequence) |
| $41 | NmiState | Current NMI state ($00–$0D) |
| $42 | MenuSelectionIndex | Current menu selection |
| $43–$4A | — | State/temp variables |
| $44 | NmiSequenceStep | Current NMI sequence step |
| $45 | — | PPU update index |
| $47 | — | Optional update flag |
| $48 | NmiSyncState | $FF=PENDING, $01=COMPLETE |
| $49 | InputActionLatch | Menu input latch |
| $A4 | RiderMotionState | Rider motion state ($01=disabled, $02=active, $03=transition, $04=complete) |
| $0100 | — | DMA/APU scratch |
| $0200 | OamBuffer | OAM shadow / sprite DMA source |
| $0300–$0301 | — | PPU update command structure pointer |
| $03B1–$03B2 | — | Timed update flag |
| $0580 | PersistentRecord0 | First persistent record (31 bytes + tails) |
| $05A3 | PersistentRecord1 | Second persistent record |
| $05CE–$05CF | ResetMagic1/2 | $A5/$5A warm-start signature |
| $05E0 | WorkingBuffer | Working buffer for serial I/O |
| $06E0 | PersistentBuffer | Persistent buffer (256 bytes) |
| $07FF | — | Audio DMA trigger |

---

## 12. PPU / Rendering

- **OAM DMA**: Every NMI, DMA from $0200–$02FF via `OAMDMA` ($4014).
- **PPU update streams**: Indexed pointer tables at $C0F6 (low) / $C111 (high), 27 entries. Processed by `ProcessPpuUpdateStream` ($D3C1) during NMI.
- **Scroll**: `PpuControlShadow` ($10) and `PpuMaskShadow` ($11) are software mirrors, updated during NMI and written to $2000/$2001.
- **OAM shadow**: Filled with $F8 (offscreen) by `FillOamShadowWithF8` during runtime PPU init.

---

## 13. Summary of Execution Flow

```
Power On / Reset
    │
    ▼
Reset ($C184) ────────────────────────────────────────┐
    │ sei, cld, disable PPU/NMI, stack=$FF            │
    │ wait 2× VBlank (PPU warm-up)                    │
    │ check reset magic ($A5,$5A at $05CE/$05CF)      │
    │   valid → warm start (preserve more RAM)        │
    │   invalid → cold start (clear more RAM)         │
    │ clear RAM ($0000–$00FF or $0000–$06FF)          │
    │ if valid: InitializePersistentRam               │
    │   (copy template to both persistent records)    │
    │ disable DMC, PPUMASK=$06, PPUCTRL=$90 (NMI on)  │
    │ InitializeRuntimePpuState                       │
    │                                                 │
    ▼                                                 │
@main ($C1A4) ────────────────────────────────────────┤
    │                                                 │
    ├─ jsr ShiftRuntimeState   ($E8BC) ──────────────┐
    │   shift $18-$1F right, carry from controller    │
    │                                                 │
    ├─ lda NmiSyncState ($48)                          │
    ├─ bmi @main      (if $FF = PENDING, loop)        │
    ├─ beq @main      (if $00, loop)                  │
    │   (only proceed when NMI set it to $01)         │
    │                                                 │
    ├─ jsr DisableNmi     ($C362)                     │
    ├─ if NmiSyncState == $01:                        │
    │     jsr TransmitRuntimeDataCode ($C6C8)         │
    │   else:                                         │
    │     jsr ReceiveRuntimeDataCode ($C739)          │
    │                                                 │
    ├─ lda #NMI_SYNC_PENDING ($FF)                    │
    │   sta NmiSyncState                              │
    ├─ jsr WaitForVblankEndAndEnableNmi               │
    └─ jmp @main   ◄──────────────────────────────────┘
                                                        │
NMI ($C23B) [once per frame]                            │
    │                                                 │
    ├─ jsr DisableNmi   (PPUCTRL bit7 = 0)            │
    ├─ PPUMASK = PpuMaskShadow \| $1A (if $45==0)     │
    ├─ OAM DMA ($0200-$02FF → $4014)                  │
    ├─ jsr NmiPpuUpdate ($C27E)  [PPU update stream]  │
    ├─ if $47 != 0:                                     │
    │     jsr UpdateRiderLanePositionStates           │
    │     jsr NmiRuntimeUpdate ($D14E)                │
    ├─ if $03B1|$03B2 == 0: inc $3F, jsr UpdateRuntimeCounters │
    ├─ jsr ReadControllers ($E8E5)                    │
    ├─ jsr UpdateAudio ($F8DE)  [4 channels + SFX]    │
    ├─ jsr DispatchNmiState ($C2A9)                   │
    │     ├─ NmiDispatchMode==0: direct state table   │
    │     │   $00=NmiState00 (menu/title)             │
    │     │   $01=$C395 (menu horiz)                  │
    │     │   $02=$C99A (UpdateGameplayRuntime)       │
    │     │   $03=DispatchNmiSequenceStep             │
    │     │   $04/$07=NmiState04_07                   │
    │     │   $05=$C99A (UpdateGameplayRuntime)       │
    │     │   $06=DispatchNmiSequenceStep             │
    │     │   $07=NmiState04_07                       │
    │     │   $08=$C3BD (menu vert)                   │
    │     │   $09=$C8D6 (UpdateRaceRuntime)           │
    │     │   $0A=$C99A (UpdateGameplayRuntime)       │
    │     │   $0B=DispatchNmiSequenceStep             │
    │     │   $0C=$C43D (serial handshake)            │
    │     │   $0D=$C41C (serial handshake)            │
    │     └─ NmiDispatchMode!=0: sequence dispatch    │
    │         (9 sequences at $C03C-$C07F)            │
    │                                                 │
    ├─ read PPUSTATUS                                 │
    ├─ WaitForVblankEndAndEnableNmi                   │
    │   (set NmiSyncState = $01 implicitly via        │
    │    main loop's view of completion)              │
    └─ rti                                            │

Serial I/O (from main loop, once per frame)
    ├─ TransmitRuntimeDataCode ($C6C8)  [when sync complete]
    │     ├─ 64-byte block from $0060                 │
    │     └─ 20-byte block from $05E0 (working buffer)│
    └─ ReceiveRuntimeDataCode ($C739)  [alternate]
          ├─ validate 5-byte header ($C17F)           │
          ├─ receive second block                     │
          └─ checksum verify                          │

Audio (from NMI via UpdateAudio at $F8DE)
    ├─ Pulse 1 channel ($4000-$4003)                 │
    ├─ Pulse 2 channel ($4004-$4007)                 │
    ├─ Noise channel ($400C-$400F)                   │
    ├─ Sound effect                                 │
    └─ DMC load if $07FF != 0                        │
```

---

## 14. Notes

- **Build verification:** `make` in `copilot/` produces `reconstructed_prg.bin` that is **byte-identical** to `prg.bin` and to the PRG bank of `rom.nes`.
- Vectors confirm NMI at $C23B, RESET at $C184, IRQ at $FFF0 (points into audio data region — likely a placeholder or unused).
- The main loop (`@main` at $C1A4) is strictly NMI-synchronized via `NmiSyncState` ($48): it loops until the NMI sets the state to $01 (COMPLETE), performs serial I/O, resets to $FF (PENDING), waits for VBlank, and repeats.
- Serial I/O uses controller port $4016 for pulse-length encoded data transmission/reception — likely for the Famicom 3D System or a similar serial peripheral.
- The NMI state machine supports both direct state dispatch (14 states via table at $C000) and sequence dispatch (9 sequences via tables at $C03C–$C07F).
- Many routines carry address-based labels (`NmiState04_07`, `UpdateSideScrollingGameplay_C073`, etc.) reflecting the automated first-pass disassembly; semantic naming is provisional.
- `symbols.inc` (embedded at the top of `prg.s`) provides NES hardware register names and key zero-page symbols.
- One build fix was required: an undefined branch target `LookupTableEntry_FAF5` was replaced with a PC-relative `bne *-$1B` to produce the original `d0 e3` bytes.