Improve ISX compatibility

Core/gb.c

@@ -559,7 +559,12 @@ int GB_load_isx(GB_gameboy_t *gb, const char *path)
 
                 READ(address);
                 address = LE16(address);
-                address &= 0x3FFF;
+                if (bank) {
+                    address &= 0x3FFF;
+                }
+                else {
+                    address &= 0x7FFF;
+                }
 
                 READ(length);
                 length = LE16(length);
@@ -710,6 +715,17 @@ done:;
         }
     }
 
+    // Inject a correct checksum, the official linker doesn't always fix it, which breaks the official boot ROMs
+    uint8_t original_checksum = gb->rom[0x14d];
+    gb->rom[0x14d] = 0;
+    for (unsigned addr = 0x0134; addr <= 0x014C; addr++) {
+        gb->rom[0x14d] -= gb->rom[addr] + 1;
+    }
+
+    if (original_checksum != gb->rom[0x14d]) {
+        GB_log(gb, "This ROM's header checksum has been automatically corrected\n");
+    }
+
     if (old_rom) {
         free(old_rom);
     }