Verify Nintendo logo checksum.

Handle read errors when loading the cartridge.

src/cartridge.ml

@@ -10,27 +10,46 @@ type memory_bank_controller =
 type t = {
   nintendo_logo : bytes;
   title : string;
-  mem_type : memory_bank_controller;
+(*  mem_type : memory_bank_controller;
   rom_size : int;
   ram_size : int;
   header_checksum : bytes;
-  global_checksum : bytes;
+  global_checksum : bytes; *)
 }
 
+(** The Nintendo logo is a checksum in the header *)
+let check_nintendo_logo cartridge =
+  let reference = Bytes.of_string
+  "\xCE\xED\x66\x66\xCC\x0D\x00\x0B\x03\x73\x00\x83\x00\x0C\x00\x0D\
+  \x00\x08\x11\x1F\x88\x89\x00\x0E\xDC\xCC\x6E\xE6\xDD\xDD\xD9\x99\
+  \xBB\xBB\x67\x63\x6E\x0E\xEC\xCC\xDD\xDC\x99\x9F\xBB\xB9\x33\x3E" in
+  (Bytes.compare cartridge.nintendo_logo reference) == 0
+
+exception Read_error
+
 let read_cartridge file =
-  print_endline file;
-  let fd = openfile file [Unix.O_RDONLY] 0o644 in
+  try
+    let fd = openfile file [Unix.O_RDONLY] 0o644 in
 
-  let _ = lseek fd 0x0104 SEEK_SET in
+    try
+      let nin_logo_offset = 0x0104 in
+      if (lseek fd nin_logo_offset SEEK_SET) != nin_logo_offset
+      then raise Read_error;
 
-  let n_logo = Bytes.create 48 in
-  let _ = read fd n_logo 0 48 in
-  print_endline "Nintendo logo:";
-  Hexa.print_bytes n_logo ~width:16;
+      let nin_logo_size = 48 in
+      let nintendo_logo = Bytes.create nin_logo_size in
+      if (read fd nintendo_logo 0 nin_logo_size) != nin_logo_size
+      then raise Read_error;
 
-  let title_b = Bytes.create 16 in
-  let _ = read fd title_b 0 16 in
-  let title = Bytes.to_string title_b in
-  printf "ROM title: %s\n" title;
+      let title_size = 16 in
+      let title_b = Bytes.create title_size in
+      if (read fd title_b 0 title_size) != title_size
+      then raise Read_error;
+      let title = Bytes.to_string title_b in
 
-  Unix.close fd
+      Unix.close fd;
+      Some { nintendo_logo; title }
+
+    with Read_error -> Unix.close fd; None
+
+  with Unix_error(err, cmd, msg) -> None

src/oboy.ml

@@ -1,4 +1,15 @@
+(** Power up sequence *)
+let power_up cartridge =
+  (** Nintendo logo scrolling *)
+  if not (Cartridge.check_nintendo_logo cartridge)
+  then print_endline "Invalid ROM."
+  else print_endline "Valid ROM."
+
 let () =
   if Array.length Sys.argv < 2
-  then print_endline "Please specify a ROM."
-  else Cartridge.read_cartridge Sys.argv.(1)
+  then print_endline "Please specify a ROM.";
+
+  let cartridge = Cartridge.read_cartridge Sys.argv.(1) in
+  match cartridge with
+  | None -> print_endline "Invalid ROM file."
+  | Some c -> power_up c