feat(gameboy): implement native Game Boy DMG/GBC sound driver, target, and CI workflow

.github/workflows/build-gameboy.yml

@@ -0,0 +1,60 @@
+name: Game Boy Build
+
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - 'targets/gameboy/**'
+      - 'backends/**'
+      - 'core/**'
+      - 'drivers/gb_sound/**'
+      - '.github/workflows/build-gameboy.yml'
+  pull_request:
+    branches: [ main ]
+    paths:
+      - 'targets/gameboy/**'
+      - 'backends/**'
+      - 'core/**'
+      - 'drivers/gb_sound/**'
+      - '.github/workflows/build-gameboy.yml'
+  workflow_dispatch:
+
+jobs:
+
+  build-gameboy:
+    name: Game Boy ROM (GBDK-2020)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install GBDK-2020
+        run: |
+          wget -q https://github.com/gbdk-2020/gbdk-2020/releases/download/4.5.0/gbdk-linux64.tar.gz
+          tar -xzf gbdk-linux64.tar.gz
+          echo "GBDKDIR=$GITHUB_WORKSPACE/gbdk/" >> $GITHUB_ENV
+          echo "$GITHUB_WORKSPACE/gbdk/bin" >> $GITHUB_PATH
+
+      - name: Build Game Boy ROM
+        run: |
+          make -C targets/gameboy/
+
+      - name: Verify Game Boy ROM Output
+        run: |
+          if [ -f targets/gameboy/out/rom.gb ] && [ -f targets/gameboy/out/rom.gbc ]; then
+            echo "Successfully generated rom.gb and rom.gbc!"
+            ls -la targets/gameboy/out/
+          else
+            echo "Error: ROM output missing!"
+            ls -la targets/gameboy/out/ || true
+            false
+          fi
+
+      - name: Upload Game Boy ROM
+        uses: actions/upload-artifact@v4
+        with:
+          name: RDJ-gameboy-rom
+          path: |
+            targets/gameboy/out/rom.gb
+            targets/gameboy/out/rom.gbc

.github/workflows/build-genesis.yml

@@ -8,6 +8,7 @@ on:
       - 'backends/**'
       - 'core/**'
       - 'drivers/ym2612/**'
+      - 'drivers/gb_sound/**'
       - 'tests/**'
       - '.github/workflows/build-genesis.yml'
   pull_request:
@@ -17,6 +18,7 @@ on:
       - 'backends/**'
       - 'core/**'
       - 'drivers/ym2612/**'
+      - 'drivers/gb_sound/**'
       - 'tests/**'
       - '.github/workflows/build-genesis.yml'
   workflow_dispatch:
@@ -82,10 +84,12 @@ jobs:
             tests/test_main.c \
             drivers/ym2612/ym2612_rdj.c \
             drivers/nes_2a03/2a03_rdj.c \
+            drivers/gb_sound/gb_rdj.c \
             backends/backend_null.c \
             backends/backend_vgm.c \
             backends/backend_serial.c \
             backends/backend_genesis_hw.c \
+            backends/backend_gb_hw.c \
             targets/genesis/src/ui.c \
             targets/genesis/src/input_mapper.c \
             -I. -Icore/ -o test_runner

.gitignore

@@ -3,9 +3,17 @@
 *.s
 !targets/nes/src/crt0.s
 out/
+targets/gameboy/out/
 test_runner
 test_output.vgm
 
+# GBDK intermediate build artifacts
+*.ihx
+*.lst
+*.sym
+*.noi
+*.map
+
 # SGDK copied sources
 targets/genesis/src/core/
 targets/genesis/src/drivers/

backends/backend_gb_hw.c

@@ -0,0 +1,105 @@
+/* GPLv3 - RetroDJ Project - Game Boy Real Hardware Backend */
+
+/*
+ * Copyright (C) 2025 RetroDJ Project
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should receive a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#if defined(RDJ_GENESIS_HARDWARE) || defined(SGDK_GCC) || defined(__m68k__)
+    #include <genesis.h>
+#elif defined(__PORT_sm83) || defined(__TARGET_gb) || defined(GBDK)
+    #include <gb/gb.h>
+    #include <stdint.h>
+    #include <stddef.h>
+#else
+    #include <stdint.h>
+    #include <stddef.h>
+#endif
+
+#include "core/rdj_backend.h"
+
+#if !defined(__PORT_sm83) && !defined(__TARGET_gb) && !defined(GBDK)
+static uint8_t gb_apu_shadow[0x40];
+#endif
+
+static void gb_hw_write_apu(uint16_t addr, uint8_t val) {
+#if defined(__PORT_sm83) || defined(__TARGET_gb) || defined(GBDK)
+    *((volatile uint8_t*)addr) = val;
+#else
+    if (addr >= 0xFF10 && addr <= 0xFF3F) {
+        gb_apu_shadow[addr - 0xFF10] = val;
+    }
+#endif
+}
+
+static void gb_hw_init(void) {
+    /* Master enable APU: Bit 7 = 1 in NR52 ($FF26) */
+    gb_hw_write_apu(0xFF26, 0x80);
+    /* Enable all output channels to left and right terminals in NR51 ($FF25) */
+    gb_hw_write_apu(0xFF25, 0xFF);
+    /* Set max stereo master volume in NR50 ($FF24) */
+    gb_hw_write_apu(0xFF24, 0x77);
+}
+
+static void gb_hw_shutdown(void) {
+    /* Disable APU master enable */
+    gb_hw_write_apu(0xFF26, 0x00);
+}
+
+static void gb_hw_write_ym2612(uint8_t port, uint8_t reg, uint8_t val) {
+    (void)port;
+    (void)reg;
+    (void)val;
+}
+
+static void gb_hw_wait_samples(uint16_t samples) {
+    (void)samples;
+}
+
+static void gb_hw_wait_frame(void) {
+#if defined(__PORT_sm83) || defined(__TARGET_gb) || defined(GBDK)
+    wait_vbl_done();
+#endif
+}
+
+static const char gb_backend_name[] = "Game Boy Real Hardware Backend";
+
+rdj_backend_t backend_gb_hw = {
+    gb_backend_name,
+    gb_hw_init,
+    gb_hw_shutdown,
+    gb_hw_write_ym2612,
+    gb_hw_write_apu,
+    gb_hw_wait_samples,
+    gb_hw_wait_frame
+};
+
+#if defined(__PORT_sm83) || defined(__TARGET_gb) || defined(GBDK)
+rdj_backend_t *rdj_active_backend = &backend_gb_hw;
+
+void rdj_set_backend(rdj_backend_t *backend) {
+    if (backend) {
+        if (rdj_active_backend && rdj_active_backend->shutdown) {
+            rdj_active_backend->shutdown();
+        }
+        rdj_active_backend = backend;
+        if (rdj_active_backend->init) {
+            rdj_active_backend->init();
+        }
+    } else {
+        rdj_active_backend = &backend_gb_hw;
+    }
+}
+#endif