265 lines
13 KiB
Markdown
265 lines
13 KiB
Markdown
# Balloon Fight (JU) — Game Flow from Power-On
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This document describes the execution flow of the disassembled Balloon Fight (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`.
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---
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## 1. Reset Vector — CPU $FFFC → $C000
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**`Reset`** ($C000–$C07E)
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On power-on or reset, the 6502 jumps to `$C000`. The reset routine performs cold-start hardware initialization:
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| Step | Code | Purpose |
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|------|------|---------|
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| 1 | `lda #$00 / sta PPUCTRL` | Disable NMI and PPU rendering |
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| 2 | `lda #$00 / sta PPUMASK` | Disable screen output |
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| 3–6 | Poll `$2002` twice | Wait for two full VBlank periods (PPU warm-up) |
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| 7 | `sei / cld` | Disable interrupts, clear decimal mode |
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| 8 | `ldx #$FF / txs` | Initialize stack pointer |
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| 9–13 | Clear Zero Page | Zero all of RAM $00–$FF |
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| 14–21 | Warm-start signature check | Compare RAM `$07FA`–`$07FC` against `"HAL"` ($C082–$C084). If valid, skip full memory clear |
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| 22–31 | Initialize memory | Clear zero page + stack page ($0700–$07FF), set `$15 = $32`, run block-init loop (`jsr $D6DE` 50 times), `jsr $C579` |
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| 32–44 | Copy initialization data | Copy 15 bytes from $C085 to $0629 (initial game state table) |
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| 45–51 | Clear local variables | Zero $03–$06 |
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| 52–56 | Copy saved signature | Re-store `"HAL"` to $07FA–$07FC |
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| 57 | `jmp MainInitialization` | Transfer control to $F1D4 |
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**Warm-start signature:** `"HAL"` stored at $C082; RAM copy at $07FA. This lets the game resume state after a soft reset without re-initializing everything.
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---
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## 2. Main Initialization
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**`MainInitialization`** is at **$F1D4** (forward-referenced in `locals.inc`). It sets up the first round, initializes object pools, and enters the main loop. Its body lives in the upper bank region (not fully shown in the decoded $C000–$C07E range).
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---
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## 3. Interrupt Vectors — CPU $FFFA–$FFFF
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Located at the end of the bank (PRG offset $3FFA):
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| Vector | Address | Handler |
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|--------|---------|---------|
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| NMI | $C094 | `NmiHandler` |
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| RESET | $C000 | `Reset` |
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| IRQ | $C0F7 | `IrqHandler` (self-loop `jmp $C0F7` — unused) |
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---
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## 4. NMI / Frame Heartbeat — `NmiHandler` ($C094)
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The NMI fires once per frame and drives the frame pacing. It:
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1. **Preserves registers** (A, X, Y on stack)
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2. **OAM DMA**: writes `$00` to `$2003`, then `$02` to `$4014` (DMA from $0200–$02FF)
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3. **PPU command buffer flush**: compares read/write offsets at $52/$53; if data pending, calls `FlushPpuCommandPreserved` ($C17C)
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4. **Transfer sources**: `jsr UpdatePpuTransferSources` ($D60D)
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5. **Build OAM**: `jsr BuildFrameOam` ($D798)
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6. **Frame counter**: `inc FrameCounter` ($19)
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7. **Scroll/rendering setup**: sets $2006/$2005 scroll registers, conditionally waits for VBlank if rendering disabled, writes $2000
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8. **Audio**: `jsr UpdateAudioFrame` ($FFF7 bank call → `Sound_F758`)
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9. **Sets `NmiCompletedFlag`** ($02 = $01)
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10. **Restores registers, `rti`**
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The main game loop runs during VBlank and waits for this flag.
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---
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## 5. Main Game Loop — $C209
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After initialization, control reaches the per-frame loop at **$C209**:
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```
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UpdateGameplayObjects ($E691) ; update all 20 gameplay-object slots
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UpdateSpecialObjectSystem ($C6F9) ; every other frame (frame counter / 2)
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[round spawner / HUD / spawning logic]
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[object physics & collision]
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[particle system]
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[OAM sorting / sprite setup]
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WaitForFrame ($F465) ; wait for NMI-completed flag
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jmp back to $C209
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```
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Key flow details:
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- **Scroll handling** ($C227): if `PpuScrollX` ($17) is non-zero, toggles `PpuControlBase` bit 0 (screen scroll page flip), decrements scroll X.
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- **Carrier/rescue sequence** ($C233): updates carrier position ($0488), target player, sound requests.
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- **Effect object pool** (8 slots, $055D–$0584): updates timers, respawns offscreen.
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- **Gameplay object pool** (20 slots, $0530–$055C): movement, position integration.
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- **Round spawner** ($C790): on frame boundaries, counts down `RoundSpawnDelay`, finds free slots, spawns round objects from tables ($C88F–$C8B5), plays spawn sound.
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- **HUD updates** ($C831): when spawn delay reaches 1, writes round HUD digits to PPU buffer.
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- **Sprite sorting**: `SortOrInitializeRecords` ($C579) maintains a sorted table of up to 50 five-byte records ($0700–$07F9) for OAM priority.
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---
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## 6. Subsystems
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### 6.1 PPU Command Buffer ($C12D–$C1C4)
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A buffered PPU update system. Commands are staged at $0300 with format:
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- Byte 0: PPU address high | $20
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- Byte 1: PPU address low
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- Byte 2: length (number of data bytes following)
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Routines:
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- `AppendBytesToPpuBuffer` ($C131) — appends raw bytes
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- `AppendLengthPrefixedPpuData` ($C139) — appends with length prefix
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- `BuildPpuCommand` ($C154, formerly `Sub_C154`) — formats address+length command
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- `FlushPpuCommandPreserved` ($C17C, formerly `Sub_C17C`) — drains buffer to $2006/$2007, preserving X/Y
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- Special handling for palette writes ($3Fxx range)
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### 6.2 Round Spawning ($C77A–$C88A)
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- `ChooseRoundSpawnIndex` ($C77A): picks a random spawn index below `RoundSpawnRandomLimit`, rejecting zero
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- `UpdateRoundSpawner` ($C790): timer-driven spawning; initializes object position, velocity (from tables at $C89F–$C8B1), flip flags, aux state; increments `SpawnSelector`; requests spawn sound ($F0 bit $04)
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- `UpdateRoundHud` / `ConfigureRoundPresentation` ($C831–$C88A): writes round number / HUD tiles to PPU buffer
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**Spawn tables** ($C88F–$C8B5): flip X/Y, X/Y offsets, parameter, velocity Y, position hi, velocity X — six entry types.
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### 6.3 Gameplay Objects ($C8B7–$C9DC)
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`UpdateRoundObjectSpritesAndPhysics` ($C8B7) updates slots 1 and 0:
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- Builds 3-sprite OAM per object from tile tables ($C9DD–$CA33)
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- Integrates fixed-point position ($C9B6)
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- Bounds testing (`TestGameplayObjectAgainstBounds`, $CA67) against rectangular regions
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- Velocity reflection on collision (`BounceGameplayObjectX/Y`, $CA4F/$CA37)
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- Object removal when offscreen
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### 6.4 Bonus Objects ($CB74–$CCCA)
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- `UpdateBonusObjectSystem` ($CB74): per-frame phase/timer advance for the single active bonus slot
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- `TestBonusObjectPlayerOverlap` ($CBB2): rectangle overlap vs both players; applies horizontal/vertical impulses, collision sound
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- `TestBonusObjectCollection` ($CC3A): phase-3 collection; starts bonus timer
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- `TriggerBonusObjectEffect` ($CCBF): increments effect counter, calls `ApplyBonusEffect` ($E983)
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- `DrawBonusObjectHud` ($CCCB): 3-row HUD tile display from phase tables ($CD26–$D046)
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### 6.5 Carrier / Rescue Sequence ($C5C3–$C715)
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- `UpdateCarrierSequence` ($C6F9): advances carrier target X while idle; finds eligible player; starts sequence
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- `FindEligiblePlayerForCarrier` ($C614): scans player slots 7→0, finds one at lower screen boundary ($B4+) matching carrier target X
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- `BeginCarrierTargetSequence` ($C6C4): captures player position/state, resets timer
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- `UpdateCarrierArrivalSequence` ($C658): moves carrier down, checks contact, transitions player to hit/rescue state, sound request
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- `UpdateCarrierObjectDisplay` ($C5C3): builds 8-sprite carrier metasprite, updates target player position relative to carrier
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- Tables: `NormalDisplayModeTable` ($C5B7), `AlternateDisplayModeTable` ($C5BB), `CarrierAnimationSequenceTable` ($C6B8)
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### 6.6 Particle System ($CD4A–$CEAD)
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- `InitializeParticleSlots` ($CD4A): 10 slots, offscreen Y ($F0)
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- `SpawnTimedParticle` ($CD5A): random timer, finds free slot, assigns position/velocity from `ParticleInitialVelocityTable` ($CEAE)
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- `UpdateParticleSystem` ($CDAA): integrates signed motion, applies scroll delta, retires offscreen particles
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- `DrawParticleSprites` ($CE2F): 3-sprite OAM per particle with animation/attribute masking
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### 6.7 Sound Driver ($FFF7 region, bank-switched)
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- `UpdateAudioFrame` jumps to `Sound_F758` in another bank
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- Sound sequence pointer table at $3BCA (32 entries)
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- Encoded sequence data $3BCA–$3FF1 (note/duration bytes with $02/$FF/$8x control codes)
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- APU channel init at $F7xx region (`InitializeApuChannelState`, `SilencePulseChannels`)
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- Sound request flags: $F0 (flags 0), $F1 (flags 1), $F2 (flags 2), $F3 (flags 3)
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---
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## 7. Memory Map (Key Zero Page / RAM)
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| Address | Name | Use |
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|---------|------|-----|
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| $00 | `PpuControlShadow` | $2000 mirror |
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| $01 | `PpuMaskShadow` | $2001 mirror |
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| $02 | `NmiCompletedFlag` | VBlank sync flag |
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| $12–$13 | `RecordIndex` / `ReverseRecordIndex` | Sorted-record insertion |
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| $14–$15 | `SpawnParameter` / `SpawnPixelPosition` | Spawn args |
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| $16 | `RenderingDisabledFlag` | Rendering state |
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| $17 | `PpuScrollX` | Scroll X |
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| $18 | `PpuControlBase` | Base PPU control |
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| $19 | `FrameCounter` | Frame tick |
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| $1B | `RandomState` | PRNG state |
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| $1D–$1E | `RecordPtrLo/Hi` | Record table pointer |
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| $23–$24 | `SpawnCommandPointerLo/Hi` | Command stream pointer |
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| $25–$26 | `DispatchPointerLo/Hi` | Spawn dispatch indirect jump |
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| $52–$53 | Ppu buffer read/write offsets | Command buffer |
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| $54–$56 | Ppu command address/length | Pending PPU command |
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| $7F | `PlayerLifecycleState` | Per-player lifecycle |
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| $88 | `PlayerSlotState` | Per-player active state |
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| $91 | `PlayerPosX` / `Player0PosX` | Player X |
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| $99 | `CarrierTargetX` | Carrier target |
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| $9A | `PlayerPosY` / `Player0PosY` | Player Y |
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| $A2 | `CarrierPosY` | Carrier Y |
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| $BD | `PlayerCollisionDisable` | Collision immunity |
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| $F0–$F3 | Sound request flags 0–3 | Audio requests |
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| $0300–$03FF | PPU command buffer | Staged PPU updates |
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| $0489–$048D | Carrier sequence state | Active, mode, target, drop, timer |
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| $0490–$0530 | Gameplay object pool (20 slots) | State, pos, velocity |
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| $055D–$0584 | Effect object pool (8 slots) | State, pos |
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| $0558–$05CE | Particle / sequence state | 10-slot particle pool |
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| $05D1–$060E | Bonus object state | Timer, phase, pos, HUD |
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| $0700–$07F9 | Sorted record table | 50 × 5-byte OAM sort records |
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| $07FA–$07FC | Warm-start signature | "HAL" |
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---
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## 8. Object Pools
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| Pool | Slots | State array | Purpose |
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|------|-------|-------------|---------|
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| Gameplay objects | 20 ($00–$13) | $0530 | Round enemies/objects, movement & physics |
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| Effect objects | 8 ($00–$07) | $055D | Short-lived spawn effects |
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| Particles | 10 ($00–$09) | $055D | Visual particles (shared state array, different offset interpretation) |
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Slot state convention: **negative ($FF) = inactive/free**, zero or positive = active.
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---
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## 9. Summary of Execution Flow
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```
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Power On / Reset
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│
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▼
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Reset ($C000) ────► hardware init, clear RAM, validate "HAL" signature
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│
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▼
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jmp MainInitialization ($F1D4)
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│
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▼
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[first-round setup, object pool init]
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│
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▼
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Main Loop ($C209) ──────────────────────────────────────────────┐
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│ │
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├─ UpdateGameplayObjects ($E691) │
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├─ UpdateSpecialObjectSystem ($C6F9) [alt frames] │
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├─ Carrier sequence ($C6F9 family) │
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├─ Round spawner ($C790) + HUD ($C831) │
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├─ Object physics & collision ($C8B7, $CA67) │
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├─ Bonus object system ($CB74) │
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├─ Particle system ($CDAA) │
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├─ OAM sort ($C579) │
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├─ WaitForFrame ($F465) ◄──────── waits for NMI flag ──────┘
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│
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▼ (loop back to $C209)
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NMI ($C094) [once per frame]
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├─ OAM DMA ($4014)
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├─ Flush PPU command buffer ($C17C)
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├─ Update transfer sources ($D60D)
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├─ Build frame OAM ($D798)
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├─ Frame counter++, scroll setup
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├─ UpdateAudioFrame ($FFF7 → Sound_F758)
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└─ Set NmiCompletedFlag, rti
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```
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---
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## 10. Notes on Provisional Areas
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Some routines in the disassembly still carry placeholder names (`Label_Cxxxx`, `Sound_FBxx`) and some data blocks were previously marked "undecoded." These have been cleaned up in this review:
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- `Sub_C154` → **`BuildPpuCommand`**
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- `Sub_C17C` → **`FlushPpuCommandPreserved`**
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- `Sub_C40C` → **`SpawnRandomObject`**
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- `SpawnCommandStreamData_C4AD` → **`RoundSpawnCommandStream`** (format documented)
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The 497 `Label_` branch targets are local intra-routine labels; none are subroutine entry points (confirmed against all `jsr`/`jmp` targets in the source). The 77 `Sound_FBxx` labels are sound-driver entry points in the banked audio region.
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Build verification: `make` in `copilot/` produces `reconstructed_prg.bin` that is **byte-identical** to `prg.bin` and to the PRG bank of `balloon.nes`. |