diff --git a/src/cartridge.ml b/src/cartridge.ml
index a3ec9ef..89aa89d 100644
--- a/src/cartridge.ml
+++ b/src/cartridge.ml
@@ -3,6 +3,8 @@ open Printf
 
 (** http://bgb.bircd.org/pandocs.htm#thecartridgeheader *)
 
+exception Read_error
+
 type memory_bank_controller =
   | ROM_ONLY
   | MBC1
@@ -10,9 +12,9 @@ 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;
+(*  ram_size : int;
   header_checksum : bytes;
   global_checksum : bytes; *)
 }
@@ -25,7 +27,22 @@ let check_nintendo_logo cartridge =
   \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
+(** ROM size is expressed in KB *)
+let get_ROM_size b =
+  match Bytes.get b 0 |> int_of_char with
+  | 0x00 ->  32 (* no ROM banking *)
+  | 0x01 ->  64 (* 4 banks *)
+  | 0x02 -> 128 (* 8 banks *)
+  | 0x03 -> 256 (* 16 banks *)
+  | 0x04 -> 512 (* 32 banks *)
+  | 0x05 -> 1000 (* 64 banks - only 63 banks used by MBC1 *)
+  | 0x06 -> 2000 (* 128 banks - only 125 banks used by MBC1 *)
+  | 0x07 -> 4000 (* 256 banks *)
+  | 0x52 -> 1100 (* 72 banks *)
+  | 0x53 -> 1200 (* 80 banks *)
+  | 0x54 -> 1500 (* 96 banks *)
+  | _ -> raise Read_error
+
 
 let read_cartridge file =
   try
@@ -47,8 +64,14 @@ let read_cartridge file =
       then raise Read_error;
       let title = Bytes.to_string title_b in
 
+      let size_b = Bytes.create 1 in
+      let _ = lseek fd 4 SEEK_CUR in
+      if (read fd size_b 0 1) != 1
+      then raise Read_error;
+      let rom_size = get_ROM_size size_b in
+
       Unix.close fd;
-      Some { nintendo_logo; title }
+      Some { nintendo_logo; title; rom_size }
 
     with Read_error -> Unix.close fd; None
 
diff --git a/src/oboy.ml b/src/oboy.ml
index a780a63..9526883 100644
--- a/src/oboy.ml
+++ b/src/oboy.ml
@@ -1,9 +1,15 @@
-(** Power up sequence *)
+open Printf
+
+(** Power up sequence
+    http://bgb.bircd.org/pandocs.htm#powerupsequence *)
 let power_up cartridge =
   (** Nintendo logo scrolling *)
   if not (Cartridge.check_nintendo_logo cartridge)
   then print_endline "Invalid ROM."
-  else print_endline "Valid ROM."
+  else
+    print_endline "Valid ROM.";
+    printf "ROM size: %iKB\n" cartridge.rom_size
+
 
 let () =
   if Array.length Sys.argv < 2