Add INT 13,41 (extensions installation check). LILO's MBR now uses

linear calls to load the MS-DOS boot sector in my test setup.

src/arch/i386/include/int13.h

 #define INT13_WRITE_SECTORS		0x03
 /** Get drive parameters */
 #define INT13_GET_PARAMETERS		0x08
+/** Extensions installation check */
+#define INT13_EXTENSION_CHECK		0x41
 /** Extended read */
 #define INT13_EXTENDED_READ		0x42
 /** Extended write */
 
 /** @} */
 
+/**
+ * @defgroup int13exts INT 13 extension flags
+ * @{
+ */
+
+/** Extended disk access functions supported */
+#define INT13_EXTENSION_LINEAR		0x01
+/** Removable drive functions supported */
+#define INT13_EXTENSION_REMOVABLE	0x02
+/** EDD functions supported */
+#define INT13_EXTENSION_EDD		0x04
+
+/** @} */
+
 extern void register_int13_drive ( struct int13_drive *drive );
 extern void unregister_int13_drive ( struct int13_drive *drive );
 extern int int13_boot ( unsigned int drive );

src/arch/i386/interface/pcbios/int13.c

 	return 0;
 }
 
+/**
+ * INT 13, 41 - Extensions installation check
+ *
+ * @v drive		Emulated drive
+ * @v bx		0x55aa
+ * @ret bx		0xaa55
+ * @ret cx		Extensions API support bitmap
+ * @ret status		Status code
+ */
+static int int13_extension_check ( struct int13_drive *drive __unused,
+				   struct i386_all_regs *ix86 ) {
+	if ( ix86->regs.bx == 0x55aa ) {
+		DBG ( "INT 13 extensions installation check\n" );
+		ix86->regs.bx = 0xaa55;
+		ix86->regs.cx = INT13_EXTENSION_LINEAR;
+		return 0;
+	} else {
+		return INT13_STATUS_INVALID;
+	}
+}
+
 /**
  * INT 13, 42 - Extended read
  *
 		case INT13_GET_PARAMETERS:
 			status = int13_get_parameters ( drive, ix86 );
 			break;
+		case INT13_EXTENSION_CHECK:
+			status = int13_extension_check ( drive, ix86 );
+			break;
 		case INT13_EXTENDED_READ:
 			status = int13_extended_read ( drive, ix86 );
 			break;