Added _directerrno() to the sim6502/sim65c02 libraries.

libsrc/sim6502/errno.s

@@ -1,11 +1,29 @@
 ;
-; Oliver Schmidt, 2013-05-16
+; 2013-05-16, Oliver Schmidt
+; 2015-07-18, Greg King
 ;
-; extern int errno;
+; Helper functions for several high-level functions.
 ;
 
         .include        "errno.inc"
 
+; ----------------------------------------------------------------------------
+; int __fastcall__ _directerrno (unsigned char code);
+; /* Set errno to a specific error code; and, return -1. Used
+; ** by the library.
+; */
+
+__directerrno:
+        jsr     __seterrno              ; Save in errno
+fail:   lda     #$FF                    ; Return -1
+        tax
+ok:     rts
+
+
+; ----------------------------------------------------------------------------
+;
+; extern int _errno;
+;
         .bss
 
 __errno: