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, 10 insertions, 0 deletions

diff --git a/src/gb.h b/src/gb.h
index 5192e85..7162b4d 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -118,10 +118,20 @@ struct GB {
     int  serial_out_fd; // if >=0, transmitted serial bytes written here (stdout)
     int  serial_in_fd;  // if >=0, received serial bytes read here (stdin), else 0xFF
     bool poweroff;      // set by the $ED opcode: request a clean emulator exit
+
+    // Boot ROM (BIOS). When boot_rom is non-NULL, gb_reset starts execution
+    // inside it (PC=0) with power-on register state; the ROM maps over low
+    // memory until the game writes 0xFF50 to hand control to the cartridge.
+    u8  *boot_rom;
+    int  boot_rom_len;    // 0x100 = DMG boot rom, 0x900 = CGB boot rom
+    bool boot_rom_active; // true while the boot ROM is still mapped in
 };
 
 void gb_init(GB *gb);
 void gb_reset(GB *gb);
+// Load a boot ROM (BIOS) image so the next gb_reset boots through it. Returns
+// 0 on success. 256-byte images boot as DMG; 2304-byte images as CGB.
+int  gb_load_bootrom(GB *gb, const char *path);
 // Run one full CPU instruction (advancing all subsystems). Returns T-cycles.
 int  gb_step(GB *gb);
 void gb_run_frame(GB *gb);