diff --git a/src/cartridge.ml b/src/cartridge.ml
index ec83073..1ddc521 100644
--- a/src/cartridge.ml
+++ b/src/cartridge.ml
@@ -10,6 +10,7 @@ type memory_bank_controller =
   | MBC1
 
 type t = {
+  full_rom : bytes;
   nintendo_logo : bytes;
   title : string;
 (*  mem_type : memory_bank_controller; *)
@@ -28,8 +29,7 @@ let check_nintendo_logo cartridge =
   (Bytes.compare cartridge.nintendo_logo reference) == 0
 
 (** ROM size is expressed in KB *)
-let get_ROM_size b =
-  match Bytes.get b 0 |> int_of_char with
+let get_ROM_size = function
   | 0x00 -> 32 (* no ROM banking *)
   | 0x01 -> 64 (* 4 banks *)
   | 0x02 -> 128 (* 8 banks *)
@@ -44,8 +44,7 @@ let get_ROM_size b =
   | _ -> raise Read_error
 
 (** RAM size is expressed in KB *)
-let get_RAM_size b =
-  match Bytes.get b 0 |> int_of_char with
+let get_RAM_size = function
   | 0x00 -> 0
   | 0x01 -> 2
   | 0x02 -> 8
@@ -55,41 +54,39 @@ let get_RAM_size b =
 
 let read_cartridge file =
   try
-    let fd = openfile file [Unix.O_RDONLY] 0o644 in
+    let ic = open_in_bin file in
+
+    let file_size = in_channel_length ic in
+
+    let full_rom = Bytes.create file_size in
+    really_input ic full_rom 0 file_size;
+    close_in ic;
 
     try
-      let nin_logo_offset = 0x0104 in
-      if (lseek fd nin_logo_offset SEEK_SET) != nin_logo_offset
-      then raise Read_error;
 
       (* Nintendo logo *)
+      let nin_logo_offset = 0x0104 in
       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 nintendo_logo = Bytes.sub full_rom nin_logo_offset nin_logo_size in
 
       (* Title *)
+      let title_offset = 0x0134 in
       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_b = Bytes.sub full_rom title_offset title_size in
       let title = Bytes.to_string title_b in
 
       (* ROM size *)
-      let byte = Bytes.create 1 in
-      let _ = lseek fd 4 SEEK_CUR in
-      if (read fd byte 0 1) != 1
-      then raise Read_error;
-      let rom_size = get_ROM_size byte in
+      let rom_size_code = Bytes.get full_rom 0x0148 |> int_of_char in
+      let rom_size = get_ROM_size rom_size_code in
 
       (* RAM size *)
-      if (read fd byte 0 1) != 1
-      then raise Read_error;
-      let ram_size = get_RAM_size byte in
+      let ram_size_code = Bytes.get full_rom 0x0149 |> int_of_char in
+      let ram_size = get_RAM_size ram_size_code in
 
-      Unix.close fd;
-      Some { nintendo_logo; title; rom_size; ram_size }
+      Some { full_rom; nintendo_logo; title; rom_size; ram_size }
 
-    with Read_error -> Unix.close fd; None
+    with
+    | Invalid_argument(msg) -> None (* This is triggered by Bytes.sub *)
+    | Read_error -> None
 
-  with Unix_error(err, cmd, msg) -> None
+  with Sys_error(msg) -> None