gb: boot ROM (BIOS) support

Add optional boot ROM emulation. When a boot ROM image is loaded, gb_reset
starts execution at PC=0 with power-on register state instead of the
documented post-boot state; the ROM overlays low memory (with the CGB
0x0100-0x01FF header hole showing the cartridge through) until the game
writes 0xFF50 to hand off. gb_load_bootrom() accepts 256-byte (DMG) or
2304-byte (CGB) images.

1 files changed, 56 insertions, 7 deletions

diff --git a/src/gb.c b/src/gb.c
index fc19e25..2aba2a1 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -4,6 +4,7 @@
 #include "cpu.h"
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 
 void gb_request_interrupt(GB *gb, u8 flag) {
@@ -47,6 +48,14 @@ static void hdma_do_block(GB *gb) {
 
 // ---------- Bus ----------
 u8 gb_read(GB *gb, u16 addr) {
+    // Boot ROM overlays low memory until the game disables it via 0xFF50.
+    // CGB layout leaves a hole at 0x0100-0x01FF where the cart header shows
+    // through (the boot logo check reads it); the ROM resumes at 0x0200.
+    if (gb->boot_rom_active) {
+        if (addr < 0x0100) return gb->boot_rom[addr];
+        if (gb->boot_rom_len > 0x0100 && addr >= 0x0200 && addr < gb->boot_rom_len)
+            return gb->boot_rom[addr];
+    }
     if (addr < 0x8000) return cart_read(&gb->cart, addr);
     if (addr < 0xA000) return ppu_read(gb, addr);
     if (addr < 0xC000) return cart_read(&gb->cart, addr);
@@ -153,6 +162,7 @@ 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 == 0xFF50) { if (val) gb->boot_rom_active = false; return; } // unmap BIOS
     if (addr >= 0xFF10 && addr <= 0xFF3F) { gb->apu[addr - 0xFF10] = val; return; }
     if (addr >= 0xFF40 && addr <= 0xFF4B) {
         if (addr == 0xFF46) { oam_dma(gb, val); return; }
@@ -215,20 +225,59 @@ void gb_hdma_hblank(GB *gb) {
 }
 
 void gb_init(GB *gb) {
+    u8 *saved_boot = gb->boot_rom;      // preserve a boot ROM loaded pre-init
+    int saved_len  = gb->boot_rom_len;
     memset(gb, 0, sizeof(*gb));
+    gb->boot_rom = saved_boot;
+    gb->boot_rom_len = saved_len;
     gb_reset(gb);
 }
 
+int gb_load_bootrom(GB *gb, const char *path) {
+    FILE *f = fopen(path, "rb");
+    if (!f) { perror("open boot rom"); return -1; }
+    fseek(f, 0, SEEK_END);
+    long n = ftell(f);
+    fseek(f, 0, SEEK_SET);
+    if (n != 0x100 && n != 0x800 && n != 0x900) {
+        fprintf(stderr, "boot rom '%s': unexpected size %ld (want 256 for DMG "
+                "or 2304 for CGB)\n", path, n);
+        fclose(f);
+        return -1;
+    }
+    free(gb->boot_rom);
+    gb->boot_rom = malloc(n);
+    if (!gb->boot_rom || fread(gb->boot_rom, 1, n, f) != (size_t)n) {
+        fclose(f); return -1;
+    }
+    gb->boot_rom_len = (int)n;
+    fclose(f);
+    return 0;
+}
+
 void gb_reset(GB *gb) {
     CPU *c = &gb->cpu;
-    gb->cgb_mode = gb->cart.cgb;
-    // post-boot register state
-    c->pc = 0x0100;
+    bool boot = (gb->boot_rom != NULL);
+    // A CGB boot ROM implies a CGB console; otherwise follow the cartridge.
+    gb->cgb_mode = boot ? (gb->boot_rom_len > 0x100) : gb->cart.cgb;
     c->sp = 0xFFFE;
-    if (gb->cgb_mode) {
-        c->af = 0x1180; c->bc = 0x0000; c->de = 0xFF56; c->hl = 0x000D;
+    if (boot) {
+        // Power-on state: start at 0x0000 inside the boot ROM, which sets up
+        // all registers itself before jumping to the cartridge entry point.
+        gb->boot_rom_active = true;
+        c->pc = 0x0000;
+        c->af = c->bc = c->de = c->hl = 0;
+        c->sp = 0x0000;
     } else {
-        c->af = 0x01B0; c->bc = 0x0013; c->de = 0x00D8; c->hl = 0x014D;
+        // Skip the boot ROM: jump straight to the game with the documented
+        // post-boot register state.
+        gb->boot_rom_active = false;
+        c->pc = 0x0100;
+        if (gb->cgb_mode) {
+            c->af = 0x1180; c->bc = 0x0000; c->de = 0xFF56; c->hl = 0x000D;
+        } else {
+            c->af = 0x01B0; c->bc = 0x0013; c->de = 0x00D8; c->hl = 0x014D;
+        }
     }
     c->ime = false;
     gb->serial_out_fd = -1;
@@ -238,5 +287,5 @@ void gb_reset(GB *gb) {
     gb->iff = 0xE1;
     gb->hdma_len = 0xFF;
     ppu_reset(gb);
-    gb->timer.div = 0x1800;
+    gb->timer.div = boot ? 0x0000 : 0x1800;
 }