diff --git a/src/core/parseopt.c b/src/core/parseopt.c
index 1dbfc7a..cd3b310 100644
--- a/src/core/parseopt.c
+++ b/src/core/parseopt.c
@@ -28,6 +28,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <ctype.h>
 #include <errno.h>
 #include <getopt.h>
 #include <ipxe/netdevice.h>
@@ -35,6 +36,7 @@
 #include <ipxe/settings.h>
 #include <ipxe/params.h>
 #include <ipxe/timer.h>
+#include <ipxe/keys.h>
 #include <ipxe/parseopt.h>
 #include <config/branding.h>
 
@@ -213,6 +215,7 @@
  * @ret rc		Return status code
  */
 int parse_key ( char *text, unsigned int *key ) {
+	int rc;
 
 	/* Interpret single characters as being a literal key character */
 	if ( text[0] && ! text[1] ) {
@@ -221,7 +224,17 @@
 	}
 
 	/* Otherwise, interpret as an integer */
-	return parse_integer ( text, key );
+	if ( ( rc = parse_integer ( text, key ) ) < 0 )
+		return rc;
+
+	/* For backwards compatibility with existing scripts, treat
+	 * integers between the ASCII range and special key range as
+	 * being relative special key values.
+	 */
+	if ( ( ! isascii ( *key ) ) && ( *key < KEY_MIN ) )
+		*key += KEY_MIN;
+
+	return 0;
 }
 
 /**
diff --git a/src/include/ctype.h b/src/include/ctype.h
index 0d79ecd..6fefd5d 100644
--- a/src/include/ctype.h
+++ b/src/include/ctype.h
@@ -10,6 +10,17 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 /**
+ * Check if character is ASCII
+ *
+ * @v character		Character
+ * @ret is_ascii	Character is an ASCII character
+ */
+static inline int isascii ( int character ) {
+
+	return ( character <= '\x7f' );
+}
+
+/**
  * Check if character is a decimal digit
  *
  * @v character		ASCII character
diff --git a/src/include/ipxe/keys.h b/src/include/ipxe/keys.h
index 2335691..49e65fa 100644
--- a/src/include/ipxe/keys.h
+++ b/src/include/ipxe/keys.h
@@ -54,6 +54,9 @@
  * The names are chosen to match those used by curses.  The values are
  * chosen to facilitate easy conversion from a received ANSI escape
  * sequence to a KEY_XXX constant.
+ *
+ * Note that the values are exposed to iPXE commands via parse_key()
+ * and therefore may not be changed without breaking existing scripts.
  */
 
 /**
@@ -79,7 +82,8 @@
  * @v terminator	ANSI escape sequence terminating character
  * @ret key		Key value
  */
-#define KEY_ANSI( n, terminator ) ( KEY_MIN + ( (n) << 8 ) + (terminator) )
+#define KEY_ANSI( n, terminator ) \
+	( KEY_MIN + ( ( (n) + 1 ) << 8 ) + (terminator) )
 
 /**
  * Extract ANSI escape sequence numeric portion
@@ -87,7 +91,7 @@
  * @v key		Key value (or relative key value)
  * @ret n		ANSI escape sequence numeric portion, or 0 for none
  */
-#define KEY_ANSI_N( key ) ( ( (key) >> 8 ) & 0xff )
+#define KEY_ANSI_N( key ) ( ( ( (key) >> 8 ) & 0xff ) - 1 )
 
 /**
  * Extract ANSI escape sequence terminating character