# Lode Runner (U) — Game Flow from Power-On

This document describes the execution flow of the disassembled Lode Runner (U) 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.

---

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

**`Reset`** ($C000–$C029)

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

| Step | Code | Purpose |
|------|------|---------|
| 1–2 | `sei` / `cld` | Disable interrupts, clear decimal mode |
| 3–4 | `ldx #$FF` / `bne Label_C00A` | X=$FF, branch always taken (delay slot?) |
| 5–6 | `txs` / `jmp $C012` | Initialize stack pointer to $01FF |
| 7–8 | `ldy #$00` / `sty Zp00` / `sty Zp01` | Zp00/Zp01 = $0000 (RAM clear pointer) |
| 9 | `tya` / `ldx #$08` | A=0, X=8 (8 pages to clear) |
| 10–14 | `Label_C01B`: `sta (Zp00),y` / `iny` / `bne` / `inc Zp01` / `dex` / `bne` | Clear $0000–$07FF (8 pages of 256 bytes) |
| 15 | `lda #$23` / `sta ZpE5` | ZpE5 ($E5) = $23 |
| 16 | `jmp $C3A6` | Jump to main initialization |

**No warm-start signature check** — full 2 KB clear every reset.

---

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

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

| Vector | Address | Handler |
|--------|---------|---------|
| NMI | $C02C | `NmiHandler` |
| RESET | $C000 | `Reset` |
| IRQ | $FFF9 | `IrqHandler` (single `rti` — unused) |

---

## 3. NMI / Frame Heartbeat — `NmiHandler` ($C02C)

The NMI fires once per frame and implements a two-path structure controlled by **Zp10** ($10):

```
NmiHandler ($C02C):
    pha / txa / pha / tya / pha           ; preserve A,X,Y
    jsr $C27C                             ; common NMI setup
    lda Zp10 ($10)
    beq Label_C050                        ; Zp10=0: Path A
    lda #$00 / sta Zp10                   ; clear Zp10
    lda Zp11 ($11) / tax
    beq Label_C043                        ; Zp11=0: Path B
    bne Label_C050                        ; Zp11≠0: Path A
Label_C043:
    jsr $C18E                             ; Path B: PPU/game update
    lda #$20 / ldx #$00 / jsr $C178       ; PPU write ($2000 + X=0)
    jmp $C0A4                             ; → main NMI exit
Label_C050:                               ; Path A: Palette/sprite update
    lda #$3F / ldx #$08 / jsr $C178       ; PPUADDR=$3F08 (palette)
    inc Zp9E ($9E)                        ; frame sub-counter
    lda Zp9E / and #$1F / bne Label_C07B  ; every 32 frames
    inc Zp9F ($9F)                        ; secondary counter
    lda Zp9F / and #$03 / tax             ; 0-3 selector
    lda #$0F / jsr $C006                  ; PPUDATA write
    lda $C39E,x / jsr $C006               ; palette data from table
    lda #$00 / jsr $C006                  ; clear
    lda #$30 / jsr $C006                  ; $30
Label_C07B:
    lda Zp9E / and #$07 / bne Label_C0A4  ; every 8 frames
    lda #$3F / ldx #$0C / jsr $C178       ; PPUADDR=$3F0C
    inc Zp72 ($72)                        ; tertiary counter
    lda Zp72 / and #$03 / tax             ; 0-3 selector
    lda #$0F / jsr $C006
    lda #$38 / jsr $C006
    lda #$28 / jsr $C006
    lda $C3A2,x / jsr $C006               ; sprite palette data
Label_C0A4:                               ; Common NMI exit
    lda Zp03 ($03) / sta $2001 (PPUMASK)  ; apply PPUMASK
    jsr $C1E1                             ; sprite/OAM update
    lda ZpDB ($DB)
    beq Label_C0D1                        ; if 0: skip gameplay
    lda Zp06 ($06) / and #$10 / beq Label_C0D1  ; bit 4 check
    jsr $C2C6 / jsr $DE9F / jsr $C1D7     ; gameplay logic
    jmp Label_C127                        ; rti
Label_C0BF:                               ; Alternate entry (ZpDB=0?)
    jsr $C1E1
    lda Zp06 / and #$10 / beq Label_C0BF  ; wait for bit 4
    jsr $DE9F / jsr $C1D7 / jsr $C2B3
    jmp Label_C127
Label_C0D1:
    lda ZpB7 ($B7)
    beq Label_C127                        ; if 0: rti
    jsr $E271                             ; level/game init
    inc ZpB9 / lda ZpB9 / cmp ZpB8 / bcc Label_C127
    lda #$00 / sta ZpB9                   ; reset counter
    ... (level transition logic)
Label_C127:
    pla / tay / pla / tax / pla / rti     ; restore, return
```

**Two NMI Modes:**
- **Path A (Zp10≠0):** Palette/sprite updates, periodic palette cycling (every 8/32 frames), sprite DMA
- **Path B (Zp10=0, Zp11=0):** `jsr $C18E` → PPU register update → main exit

**Key ZP Flags:**
- `Zp10` ($10): NMI path selector (0=Path B, non-zero=Path A)
- `Zp11` ($11): Sub-path selector for Path B
- `Zp9E` ($9E): Frame sub-counter (palette cycling)
- `Zp9F` ($9F): Secondary frame counter
- `Zp72` ($72): Tertiary counter (sprite palette)
- `ZpDB` ($DB): Gameplay active flag
- `Zp06` ($06): Bit 4 = gameplay trigger
- `ZpB7` ($B7): Level transition flag
- `Zp03` ($03): PPUMASK shadow

---

## 4. Main Initialization — `$C3A6` (from Reset)

After reset jumps to `$C3A6`, the game initializes title screen and enters main loop. The main loop is driven by NMI synchronization — the main thread waits for NMI to signal frame completion via ZP flags.

Key initialization routines (reached from `$C3A6`):
- `$C178`: PPU address/data write helper (A=addr high, X=addr low → $2006/$2006/$2007)
- `$C18E`: PPU control update (reads Zp03→PPUMASK, Zp10→PPUCTRL)
- `$C1E1`: Sprite/OAM update (builds OAM at $0200)
- `$C27C`: Common NMI setup (called at NMI entry)

---

## 5. Key Subsystems

### 5.1 PPU Helper — `$C178` / `$C006`
- `$C178`: Set PPUADDR from A (high) / X (low), write via `$2006` twice
- `$C006`: Write A to PPUDATA ($2007), RTS
- `$C18E`: Read PPUSTATUS, write Zp03→PPUMASK, Zp10→PPUCTRL

### 5.2 Sprite/OAM System — `$C1E1` / `$C1D7`
- Builds OAM at $0200 from object state arrays
- `$C1D7`: Called from NMI paths for per-frame sprite update

### 5.3 Gameplay Logic — `$C2C6`, `$DE9F`, `$C1D7`, `$C2B3`
- `$C2C6`: Player/input handling
- `$DE9F`: Enemy/guard AI
- `$C2B3`: Level progression, scoring

### 5.4 Level Data & Transitions — `$E271`, `$C0E4`–`$C123`
- `$E271`: Level load/init
- `$C0E4`: State machine on `ZpBE`/`ZpBA` (pointer to level data)
- Level data tables at `$C2F0`, `$C378`, `$C39E`, `$C3A2`

### 5.5 Audio — `$C12D` / `$C143` / `$C16E`
- APU register writes ($4000–$4003) for pulse channels
- Frequency data from `$C2F0`, `$C378`
- Called from level transition / gameplay

---

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

| Address | Name | Use |
|---------|------|-----|
| $00–$01 | `Zp00`/`Zp01` | 16-bit pointer (RAM clear, PPU) |
| $03 | `Zp03` | PPUMASK shadow |
| $06 | `Zp06` | Gameplay flags (bit 4 = trigger) |
| $10 | `Zp10` | NMI path selector (0=title, ≠0=game) |
| $11 | `Zp11` | NMI sub-path selector |
| $12–$13 | `Zp12`/`Zp13` | Object/sprite pointers |
| $20–$21 | `Zp20`/`Zp21` | Player position (X/Y) |
| $26 | `Zp26` | Player state/animation |
| $28 | `Zp28` | Player velocity |
| $32–$35 | `Zp32`–`Zp35` | Enemy/guard positions |
| $39 | `Zp39` | Level timer |
| $52–$5F | `Zp52`–`Zp5F` | Sprite attribute buffers |
| $72 | `Zp72` | Sprite palette cycle counter |
| $85–$86 | `Zp85`/`Zp86` | Level data pointers |
| $9E–$9F | `Zp9E`/`Zp9F` | Frame counters (palette cycling) |
| $B7 | `ZpB7` | Level transition trigger |
| $B8–$B9 | `ZpB8`/`ZpB9` | Level transition counters |
| $BA–$BF | `ZpBA`–`ZpBF` | Level data pointer + state |
| $DB | `ZpDB` | Gameplay active flag |
| $E5 | `ZpE5` | Init constant ($23) |
| $0200 | OAM buffer | 64 sprites (DMA source) |
| $0300+ | Game state arrays | Player, enemies, level data |

---

## 7. Data Tables

| Table | Address | Description |
|-------|---------|-------------|
| Palette data (BG) | $C39E | 4 entries, indexed by Zp9F&3 |
| Palette data (sprites) | $C3A2 | 4 entries, indexed by Zp72&3 |
| Audio frequency low | $C2F0 | Note→period low byte |
| Audio frequency high | $C378 | Note→period high byte (×2 + index) |
| Level data pointer | `ZpBA`/`ZpBC` | Indirect pointer to level scripts |

---

## 8. Summary of Execution Flow

```
Power On / Reset
    │
    ▼
Reset ($C000) ────► sei/cld, stack=$FF
    │
    ├─ Clear $0000–$07FF (8 pages via Zp00/Zp01)
    │
    ├─ ZpE5 = $23
    │
    ▼
jmp $C3A6 (Main Initialization)
    │
    ├─ Title screen setup
    ├─ PPU/APU init
    └─ Enter main loop (VBlank synchronized)

Main Loop (implicit, VBlank-driven)
    │
    ▼ (NMI fires once per frame)

NMI ($C02C)
    ├─ Preserve A,X,Y
    ├─ jsr $C27C (common setup)
    ├─ lda Zp10
    │     ├─ =0 (Title/Menu):
    │     │    lda Zp11
    │     │    ├─ =0: jsr $C18E (PPU update), $C178, jmp $C0A4
    │     │    └─ ≠0: Path A (palette cycling)
    │     │
    │     └─ ≠0 (Gameplay):
    │          lda #$00 / sta Zp10
    │          lda Zp11 / tax
    │          ├─ =0: jsr $C18E, $C178, jmp $C0A4
    │          └─ ≠0: Path A (palette cycling)
    │
    ├─ Path A (Label_C050):
    │     Palette writes to $3F08/$3F0C every 8/32 frames
    │     inc Zp9E, Zp9F, Zp72
    │
    ├─ Common Exit ($C0A4):
    │     lda Zp03 → PPUMASK
    │     jsr $C1E1 (sprite update)
    │     lda ZpDB
    │     ├─ =0: Label_C0BF (wait for Zp06 bit 4)
    │     └─ ≠0: Check Zp06 bit 4
    │           ├─ Clear: Label_C0D1 (level transition check)
    │           └─ Set: jsr $C2C6, $DE9F, $C1D7 (gameplay)
    │
    └─ Label_C127: Restore A,X,Y, rti

Gameplay (when ZpDB≠0, Zp06&$10)
    ├─ $C2C6: Player control
    ├─ $DE9F: Enemy AI
    ├─ $C1D7: Sprite update
    ├─ $C2B3: Level/scoring
    └─ Level transition: $E271, $C0E4 state machine
```

---

## 9. Naming and Residue

**Pass 1 complete** — 11,057 lines, 627 `Label_` definitions, canonical audit comments, 52.6% code.

**Pass 2 complete (2026-10-20)** — All 627 label definitions renamed. 196 semantic routine entry points named via `_tools/pass2.py` derive/apply using a hand-curated routinemap; remaining 431 internal branch targets named as `<Routine>_Block[_n]`. Zero `Label_Cxxxx` definitions remain.

**Pass 3 partial (2026-10-20)** — `_tools/apply_symbols.py` substituted **85 operands** (83 hardware registers + 2 single-name zero-page operands). Zero multi-alias residue. **2 semantic zero-page aliases applied**; remaining 150 zero-page slots retain neutral `ZpXX` names in `locals.inc`. Data tables/structures named in Memory Map section.

**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).