robustness improvements in IEC support

c64/kernal/talk.s

@@ -2,7 +2,9 @@
 
 talk:
 
-	;; This trivial routine is documented in 'Compute's Mapping the Commodore 64', page 223
+	;; This routine is documented in 'Compute's Mapping the Commodore 64', page 223 and
+	;; https://www.pagetable.com/?p=1031, , https://github.com/mist64/cbmbus_doc
+	and #$1F ; make sure bits encoding the command are cleared out
 	ora #$40
 	jmp iec_tx_command