several IEC routines now properly preserve X/Y registers

c64/kernal/second.s

@@ -4,16 +4,15 @@
 ;;
 ;; - [RG64] C64 Programmer's Reference Guide   - page 296
 ;; - [CM64] Compute's Mapping the Commodore 64 - page 223/224
+;; - https://www.pagetable.com/?p=1031, , https://github.com/mist64/cbmbus_doc
 ;;
 ;; CPU registers that has to be preserved (see [RG64]): .X, .Y
 ;;
 
-;; XXX currently does not preserve registers, to be fixed!
 
 second:
 
-	;; see also https://www.pagetable.com/?p=1031, , https://github.com/mist64/cbmbus_doc
-
 	and #$1F ; make sure bits encoding the command are cleared out
 	ora #$60
+	sta BSOUR
 	jmp iec_tx_command