apu: register-level emulation of 0xFF10-0xFF3F (fixes music fades)

The APU was a black hole: reads returned 0xFF, writes were dropped. That hangs
any game that fades music out by decrementing the NR50/rAUDVOL volume register
until it reads 0 (Pokemon's StopMusic spin-wait) - the write never stuck, the
read never hit 0, infinite loop. Store the 48 sound registers and return them
with the standard hardware read-back OR-masks (NR50 mask 0x00 = exact readback).
No synthesis, just correct register behavior.

2 files changed, 19 insertions, 2 deletions

diff --git a/src/gb.c b/src/gb.c
index 9ed2521..afdcb89 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -64,7 +64,19 @@ u8 gb_read(GB *gb, u16 addr) {
     if (addr == 0xFF02) return gb->sc | 0x7E;
     if (addr >= 0xFF04 && addr <= 0xFF07) return timer_read(gb, addr);
     if (addr == 0xFF0F) return gb->iff | 0xE0;
-    if (addr >= 0xFF10 && addr <= 0xFF3F) return 0xFF; // APU (stub)
+    if (addr >= 0xFF10 && addr <= 0xFF3F) {
+        // Register-level APU: return stored value OR'd with the bits that read
+        // back as 1 on real hardware (write-only / unused bits). NR50 (0xFF24)
+        // has mask 0x00, so it reads back exactly what was written, which is
+        // what the game's volume fade-out relies on.
+        static const u8 apu_or[0x30] = {
+            0x80,0x3F,0x00,0xFF,0xBF, 0xFF, 0x3F,0x00,0xFF,0xBF,
+            0x7F,0xFF,0x9F,0xFF,0xBF, 0xFF, 0xFF,0x00,0x00,0xBF,
+            0x00,0x00,0x70, 0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+            0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 /* FF30-FF3F wave RAM: exact */
+        };
+        return gb->apu[addr - 0xFF10] | apu_or[addr - 0xFF10];
+    }
     if (addr >= 0xFF40 && addr <= 0xFF4B) {
         if (addr == 0xFF46) return gb->dma;
         return ppu_read_reg(gb, addr);
@@ -114,7 +126,7 @@ void gb_write(GB *gb, u16 addr, u8 val) {
     }
     if (addr >= 0xFF04 && addr <= 0xFF07) { timer_write(gb, addr, val); return; }
     if (addr == 0xFF0F) { gb->iff = val & 0x1F; return; }
-    if (addr >= 0xFF10 && addr <= 0xFF3F) return; // APU stub
+    if (addr >= 0xFF10 && addr <= 0xFF3F) { gb->apu[addr - 0xFF10] = val; return; }
     if (addr >= 0xFF40 && addr <= 0xFF4B) {
         if (addr == 0xFF46) { oam_dma(gb, val); return; }
         ppu_write_reg(gb, addr, val);
diff --git a/src/gb.h b/src/gb.h
index 806daba..7439615 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -94,6 +94,11 @@ struct GB {
     // serial
     u8 sb, sc;
 
+    // APU registers 0xFF10-0xFF3F. We don't synthesize sound, but games read
+    // these back (e.g. the music fade-out decrements NR50/rAUDVOL until it
+    // reads 0), so they must behave as real storage, not a 0xFF black hole.
+    u8 apu[0x30];
+
     // DMA
     u8 dma;