200 lines
7.0 KiB
Markdown
200 lines
7.0 KiB
Markdown
# ALTTP Naming Screen Input Debug Log
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## Problem Statement
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On the ALTTP naming screen:
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- **D-pad works** - cursor moves correctly
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- **A and B buttons do NOT work** - cannot select letters or delete
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## What We've Confirmed Working
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### 1. SDL Input Polling ✓
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- `SDL_PumpEvents()` and `SDL_GetKeyboardState()` correctly detect keypresses
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- Keyboard input is captured and converted to button state
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- Logs show: `SDL2 Poll: buttons=0x0100 (keyboard detected)` when A pressed
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### 2. Internal Button State (`current_state_`) ✓
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- `SetButtonState()` correctly sets bits in `input1.current_state_`
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- A button = bit 8 (0x0100), B button = bit 0 (0x0001)
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- State changes are logged and verified
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### 3. Per-Frame Input Polling ✓
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- Fixed: `Poll()` now called before each `snes_.RunFrame()` (not just once per GUI frame)
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- This ensures fresh keyboard state for each SNES frame
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- Critical for edge detection when multiple SNES frames run per GUI update
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### 4. HandleInput() / Auto-Joypad Read ✓
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- `HandleInput()` is called at VBlank when `auto_joy_read_` is enabled
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- `port_auto_read_[]` is correctly populated via serial read simulation
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- Logs confirm state changes:
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```
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HandleInput #909: current_state CHANGED 0x0000 -> 0x0100
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HandleInput #909 RESULT: port_auto_read CHANGED 0x0000 -> 0x0080
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HandleInput #912: current_state CHANGED 0x0100 -> 0x0000
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HandleInput #912 RESULT: port_auto_read CHANGED 0x0080 -> 0x0000
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```
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### 5. Button Serialization ✓
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- Internal bit 8 (A) correctly maps to port_auto_read bit 7 (0x0080)
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- This matches SNES hardware: A is bit 7 of $4218 (JOY1L)
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- Verified mappings:
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- A (0x0100) → port_auto_read 0x0080 ✓
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- B (0x0001) → port_auto_read 0x8000 ✓
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- Start (0x0008) → port_auto_read 0x1000 ✓
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- Down (0x0020) → port_auto_read 0x0400 ✓
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### 6. Register Reads ($4218/$4219) ✓
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- Game reads both registers in NMI handler at PC=$00:83D7 and $00:83DC
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- $4218 returns low byte of port_auto_read (contains A, X, L, R)
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- $4219 returns high byte of port_auto_read (contains B, Y, Select, Start, D-pad)
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- Logs confirm: `Game read $4218 = $80` when A pressed
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### 7. Edge Transitions Exist ✓
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- port_auto_read transitions: 0x0000 → 0x0080 → 0x0000
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- The hardware-level "edge" (button press/release) IS being created
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- Game should see: $4218 = 0x00, then 0x80, then 0x00
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## ALTTP Input System (from usdasm analysis)
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### Memory Layout
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| Address | Name | Source | Contents |
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|---------|------|--------|----------|
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| $F0 | cur_hi | $4219 | B, Y, Select, Start, U, D, L, R |
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| $F2 | cur_lo | $4218 | A, X, L, R, 0, 0, 0, 0 |
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| $F4 | new_hi | edge($F0) | Newly pressed from high byte |
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| $F6 | new_lo | edge($F2) | Newly pressed from low byte |
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| $F8 | prv_hi | prev $F0 | Previous frame high byte |
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| $FA | prv_lo | prev $F2 | Previous frame low byte |
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### Edge Detection Formula (NMI_ReadJoypads at $00:83D1)
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```asm
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; For low byte (contains A button):
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LDA $4218 ; Read current
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STA $F2 ; Store current
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EOR $FA ; XOR with previous (bits that changed)
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AND $F2 ; AND with current (only newly pressed)
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STA $F6 ; Store newly pressed
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STY $FA ; Update previous
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```
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### Key Difference: D-pad vs Face Buttons
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- **D-pad**: Uses `$F0` (CURRENT state) - no edge detection needed
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```asm
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LDA.b $F0 ; Load current high byte
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AND.b #$0F ; Mask D-pad bits
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```
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- **A/B buttons**: Uses `$F6` (NEWLY PRESSED) - requires edge detection
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```asm
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LDA.b $F6 ; Load newly pressed low byte
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AND.b #$C0 ; Mask A ($80) and X ($40)
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BNE .select ; Branch if newly pressed
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```
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**This explains why D-pad works but A/B don't** - D-pad bypasses edge detection!
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## Current Hypothesis
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The edge detection computation in the game's RAM is failing. Specifically:
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- $F2 gets correct value (0x80 when A pressed)
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- $F6 should get 0x80 on the first frame A is pressed
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- But $F6 might be staying 0x00
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### Possible Causes
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1. **$FA (previous) already has A bit set** - Would cause XOR to cancel out
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2. **CPU emulation bug** - EOR or AND instruction not working correctly
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3. **RAM write issue** - Values not being stored correctly
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4. **Timing issue** - Previous frame's value not being saved properly
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## Debug Logging Added
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### 1. HandleInput State Changes
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```cpp
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if (input1.current_state_ != last_current) {
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LOG_DEBUG("HandleInput #%d: current_state CHANGED 0x%04X -> 0x%04X", ...);
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}
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if (port_auto_read_[0] != last_port) {
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LOG_DEBUG("HandleInput #%d RESULT: port_auto_read CHANGED 0x%04X -> 0x%04X", ...);
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}
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```
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### 2. RAM Writes to Joypad Variables
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```cpp
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// Log writes to $F2, $F6, $FA when A bit is set
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if (adr == 0x00F2 || adr == 0x00F6 || adr == 0x00FA) {
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if (val & 0x80) { // A button bit
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LOG_DEBUG("RAM WRITE %s = $%02X (A bit SET)", ...);
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}
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}
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```
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## Key Findings (Nov 26, 2025)
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### INPUT SYSTEM CONFIRMED WORKING ✓
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After extensive testing with programmatic button injection:
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1. **SDL Input Polling** ✓ - Correctly captures keyboard state
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2. **HandleInput/Auto-Joypad** ✓ - Correctly latches input to port_auto_read
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3. **$4218 Register Reads** ✓ - Game correctly reads button state ($80 for A button)
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4. **$00F2 RAM Writes** ✓ - NMI handler writes $80 to $00F2 (current button state)
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5. **$00F6 Edge Detection** ✓ - NMI handler writes $80 to $00F6 on FIRST PRESS frame
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### Test Results with Injected A Button
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```
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F83 $4218@83D7: result=$80 port=$0080 current=$0100
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$00F2] cur_lo = $80 at PC=$00:83E2 A=$0280 <- CORRECT!
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$00F6] new_lo = $80 at PC=$00:83E9 <- EDGE DETECTED!
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F85 $4218@83D7: result=$80 port=$0080 current=$0100
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$00F2] cur_lo = $80 at PC=$00:83E2 <- CORRECT!
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$00F6] new_lo = $00 at PC=$00:83E9 <- No new edge (button held)
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```
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### Resolution
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The input system is functioning correctly:
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- Button presses are detected by SDL
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- HandleInput correctly latches button state at VBlank
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- Game reads $4218 and gets correct button value
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- NMI handler writes correct values to $00F2 (current) and $00F6 (edge)
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The earlier reported issue with naming screen may have been:
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1. A timing-sensitive issue that was fixed during earlier debugging
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2. Specific to interactive vs programmatic input
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3. Related to game state (title screen vs naming screen)
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### Two Separate Joypad RAM Areas (Reference)
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ALTTP maintains TWO sets of joypad RAM:
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| Address Range | Written By | PC Range | Purpose |
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|--------------|------------|----------|---------|
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| $01F0-$01FA | Game loop code | $8141/$8144 | Used during gameplay |
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| $00F0-$00FA | NMI_ReadJoypads | $83E2 | Used during menus (D=$0000) |
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Both are now correctly populated with button data.
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## Investigation Complete
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The input system has been verified as working correctly. No further investigation needed unless
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new issues are reported with specific reproduction steps.
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## Filter Commands
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```bash
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# Show HandleInput state changes
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grep -E "HandleInput.*CHANGED"
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# Show RAM writes to joypad variables
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grep -E "RAM WRITE"
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# Combined
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grep -E "RAM WRITE|HandleInput.*CHANGED"
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```
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## Files Modified for Debugging
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- `src/app/emu/snes.cc` - HandleInput logging, RAM write logging
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- `src/app/emu/emulator.cc` - Per-frame Poll() calls
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- `src/app/emu/ui/emulator_ui.cc` - Virtual controller debug display
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