# Popeye (JU) (PRG 0) — Game Flow from Power-On

This document describes the execution flow of the disassembled Popeye (JU) (PRG 0) 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` at the PRG level.

**Companion note:** `Popeye (JU) (PRG 1)` holds a different PRG image of the same title (different mirroring flag: Horizontal vs Vertical). Neither is currently known to be a superset of the other.

---

## 1. Reset Vector — CPU $FFFC → $C649

**`Reset`** ($C649–$C683)

On power-on or reset, the 6502 jumps to **$C649** (non-$C000 entry, Donkey Kong Jr. style). The reset routine performs cold-start hardware initialization:

| Step | Code | Purpose |
|------|------|---------|
| 1–2 | `sei` / `cld` | Disable interrupts, clear decimal mode |
| 3–4 | `lda #$10` / `sta $2000` | PPUCTRL = $10 (NMI off, sprite $0000, BG $0000, 8×8) |
| 5–6 | `ldx #$FF` / `txs` | Initialize stack pointer to $01FF |
| 7–11 | `Label_C653`: poll `$2002` via `and #$80` / `beq` | Wait for VBlank (bit 7 of PPUSTATUS) |
| 12–13 | `ldy #$07` / `sty Zp01` / `ldy #$00` / `sty Zp00` | Zp00/Zp01 = $0700 (RAM clear pointer) |
| 14 | `tya` | A = 0 |
| 15–19 | `Label_C663`: `sta (Zp00),y` / `dey` / `bne` / `dec Zp01` / `bpl` | Clear $0000–$07FF (8 pages) |
| 20 | `jsr $C686` | Initialize player/object state at $0500–$0504 |
| 21 | `lda #$01` / `sta Zp60` | Zp60 ($60) = 1 (game state) |
| 22 | `lda #$00` / `sta Zp63` | Zp63 ($63) = 0 |
| 23 | `lda Zp10` / `eor #$80` / `sta $2000` / `sta Zp10` | PPUCTRL ^= $80 (toggle NMI enable) + shadow |
| 24 | `Label_C680`: `jsr $EE74` / `jmp $C680` | Infinite loop: waits for first NMI |

**`$C686` (Label_C686)** — Player state init:
```
    lda #$11 / sta $0500 / sta $0502
    lda #$03 / sta $0501
    lda #$01 / sta $0503
    ldx #$19 / lda #$24
Label_C69C:
    sta $0504,x   ; fill $0504–$051C with $24 (25 bytes)
    dex / bne Label_C69C
    ldx #$00
```

Then falls through to `$C6A4` (main loop entry).

---

## 2. Main Loop — `$C6A4` onward

After reset initialization, the main loop runs at `$C6A4`, synchronized with NMI via Zp10/Zp60/Zp63 flags. The loop calls various gameplay subroutines and waits for NMI to signal frame completion.

---

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

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

| Vector | Address | Handler |
|--------|---------|---------|
| NMI | $C701 | `NmiHandler` |
| RESET | $C649 | `Reset` |
| IRQ | $FFF0 | `IrqHandler` (single `sta (Zp16,x)` — undocumented opcode $81, effectively unused) |

---

## 4. NMI / Frame Heartbeat — `NmiHandler` ($C701)

The NMI fires once per frame and implements a state machine dispatching on Zp30/Zp31/Zp60/Zp63:

```
NmiHandler ($C701):
    lda Zp10 ($10) / and #$7F / sta $2000 / sta Zp10  ; PPUCTRL &= ~$80 (NMI off during frame)
    lda Zp11 ($11) / eor #$18 / sta $2001             ; PPUMASK ^= $18 (toggle bits 3,4)
    jsr $ED34                                         ; sprite/OAM update
    jsr $ED20                                         ; PPU/scroll update
    lda $0400 / bne Label_C724                        ; check $0400
    lda $0401 / bne Label_C724                        ; check $0401
    jsr $EE5E                                         ; attract mode audio
Label_C724:
    jsr $F970                                         ; gameplay audio
    jsr $EE89                                         ; sound effects
    lda Zp43 ($43)
    bne Label_C732
    lda #$FF / sta Zp43                               ; init Zp43 = $FF
Label_C732:
    lda Zp30 ($30)
    beq Label_C73C
    jsr $CA8B / jmp $C75D                             ; state: Zp30≠0
Label_C73C:
    lda Zp31 ($31)
    beq Label_C746
    jsr $CAD4 / jmp $C75D                             ; state: Zp31≠0
Label_C746:
    lda Zp60 ($60)
    bne Label_C754
    lda Zp63 ($63)
    beq Label_C75A
    jsr $CB12 / jmp $C75D                             ; state: Zp60=0, Zp63≠0
Label_C754:
    jsr $C76A / jmp $C75D                             ; state: Zp60≠0
Label_C75A:
    jsr $C89B                                         ; state: Zp60=0, Zp63=0
Label_C75D:
    ... (common NMI exit → rti)
```

**NMI State Machine:**
| Condition | Handler | Description |
|-----------|---------|-------------|
| $0400≠0 or $0401≠0 | `$EE5E` | Attract mode audio |
| Zp30≠0 | `$CA8B` | Gameplay mode A |
| Zp31≠0 | `$CAD4` | Gameplay mode B |
| Zp60≠0 | `$C76A` | Gameplay mode C |
| Zp60=0, Zp63≠0 | `$CB12` | Bonus/stage transition |
| Zp60=0, Zp63=0 | `$C89B` | Title/attract mode |

**Key NMI Operations:**
1. PPUCTRL &= $7F (disable NMI during frame), PPUMASK ^= $18
2. Sprite/OAM update (`$ED34`), PPU/scroll update (`$ED20`)
3. Audio: attract (`$EE5E`) or gameplay (`$F970` + `$EE89`)
4. State dispatch via Zp30/Zp31/Zp60/Zp63
5. Zp43 init to $FF on first gameplay NMI

---

## 5. Key Subsystems

### 5.1 Sprite/OAM System — `$ED34` / `$ED20`
- `$ED34`: Builds OAM at $0200 from object arrays
- `$ED20`: PPU scroll/register updates

### 5.2 Audio Engine
- **Attract:** `$EE5E` (called when $0400/$0401 set)
- **Gameplay:** `$F970` (music), `$EE89` (sound effects)
- APU registers at $4000–$4013

### 5.3 Gameplay Logic
- `$CA8B` / `$CAD4` / `$C76A` / `$CB12` / `$C89B`: State-specific gameplay updates
- Player (Popeye), enemies (Bluto, Sea Hag), projectiles, collectibles (spinach, hearts, letters)

### 5.4 Level Data — $C000–$C648 region
- 1609 bytes classified as `.byte` data but referenced by 148+ code instructions
- Contains level layouts, enemy paths, platform positions
- Referenced via `lda $Cxxx,x`, `bit $Cxxx` — this is **level/map data**, not code

### 5.5 Indirect Dispatch — `$ECFF` (`jmp ($0002)`)
`jmp ($0002)` at $ECFF — target address in ZP $0002/$0003, set by caller. Not followed statically.

---

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

| Address | Name | Use |
|---------|------|-----|
| $00–$01 | `Zp00`/`Zp01` | 16-bit pointer (RAM clear) |
| $02–$03 | `Zp02`/`Zp03` | Indirect dispatch pointer (`jmp ($0002)`) |
| $10 | `Zp10` | PPUCTRL shadow (bit 7 = NMI enable) |
| $11 | `Zp11` | PPUMASK shadow (toggled via EOR #$18) |
| $30–$31 | `Zp30`/`Zp31` | Gameplay state selectors |
| $43 | `Zp43` | Audio state (init $FF) |
| $60 | `Zp60` | Game state (set 1 at reset) |
| $63 | `Zp63` | Sub-state (cleared at reset) |
| $0200 | OAM buffer | Sprite DMA source |
| $0400–$0401 | Attract flags | Checked in NMI for audio path |
| $0500–$051C | Player state | Initialized at $C686 ($11,$03,$11,$24×25) |

---

## 7. Data Tables

| Table | Address | Description |
|-------|---------|-------------|
| Level/map data | $C000–$C648 | 1609 bytes of level layouts, enemy paths (referenced by code) |
| Audio sequences | $F970+ | Music/SFX data |

---

## 8. Summary of Execution Flow

```
Power On / Reset
    │
    ▼
Reset ($C649) ────► sei/cld, PPUCTRL=$10, stack=$FF
    │
    ├─ Wait VBlank (poll $2002 bit 7)
    │
    ├─ Clear $0000–$07FF (8 pages via Zp00/Zp01)
    │
    ├─ jsr $C686:
    │     $0500=$11, $0501=$03, $0502=$11, $0503=$01
    │     $0504–$051C = $24 (25 bytes)
    │
    ├─ Zp60=1, Zp63=0
    ├─ PPUCTRL ^= $80 (enable NMI) → Zp10
    │
    ▼
Label_C680: jsr $EE74 / jmp $C680  (wait for first NMI)
    │
    ▼ (NMI fires)

NMI ($C701) [once per frame]
    ├─ Zp10 &= $7F → PPUCTRL (NMI off during frame)
    ├─ Zp11 ^= $18 → PPUMASK (toggle bits 3,4)
    ├─ jsr $ED34 (sprite/OAM update)
    ├─ jsr $ED20 (PPU/scroll update)
    ├─ Check $0400/$0401:
    │     ├─ Set: jsr $EE5E (attract audio)
    │     └─ Clear:
    │          jsr $F970 (gameplay audio)
    │          jsr $EE89 (sound effects)
    │          Zp43 init ($FF)
    │          Dispatch on Zp30/Zp31/Zp60/Zp63:
    │               Zp30≠0 → $CA8B
    │               Zp31≠0 → $CAD4
    │               Zp60≠0 → $C76A
    │               Zp60=0,Zp63≠0 → $CB12
    │               Zp60=0,Zp63=0 → $C89B
    └─ rti

Main Loop ($C6A4+)
    ├─ Gameplay logic (player, enemies, collectibles)
    ├─ Collision detection
    ├─ Level progression
    └─ Loop (VBlank synchronized via Zp10/Zp60/Zp63)
```

---

## 9. Naming and Residue

**Pass 1 complete** — 14,875 lines, canonical audit comments, 72.3% code. 6 seeded routine entries from table references.

**Pass 2 complete** — all 878 `Label_` definitions renamed: 2 semantic entry names (`Reset`, `NmiHandler`) + 315 `SubC<addr>` entry points + 561 `<Routine>_Block_<addr>` internal branch targets. No `Label_` remains. Every `jsr`/`jmp` target is a named routine.

**Pass 3 complete** — 4 semantic zero-page aliases (PtrLo/Hi, Ptr2Lo/Hi) added to `locals.inc`; 79 operands substituted via `apply_symbols.py` (15 single + 64 hardware); data tables named: `LevelData` ($C000–$C648), `AudioData` ($F088–$F832), `VectorPageData` ($FCCE–$FFEF). Remaining zero-page slots retain neutral `ZpXX` names.

**Indirect dispatch** (not followed statically): `jmp ($0002)` at $ECFF.

**Large data region** — $C000–$C648 (1609 bytes) classified as data but referenced by 148+ code instructions. This is level/enemy pattern data.

**PRG 0 vs PRG 1** — Same vectors, same structure. PRG 0: Vertical mirroring. PRG 1: Horizontal mirroring. PRG bytes differ slightly (different build).

**Promotion outstanding** — Add `header.s`/`chr.s` + HEADER/PRG/CHR `linker.cfg`, switch `make` to whole-ROM `cmp`, move `_tools` paths from `../../../_tools/` to `../../_tools/`.

Build verification: `make` in `copilot/` produces `reconstructed_prg.bin`, compared against `prg.bin` (PRG byte-identical).