several IEC routines now properly preserve X/Y registers

c64/kernal/unlsn.s

@@ -4,11 +4,11 @@
 ;;
 ;; - [RG64] C64 Programmer's Reference Guide   - page 303
 ;; - [CM64] Compute's Mapping the Commodore 64 - page 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!
 
 unlsn:
 
@@ -17,15 +17,12 @@ unlsn:
 	;; First check whether we have something to be sent in the buffer
 	lda C3PO
 	beq +
-	lda BSOUR
 	sec ; send it with EOI
 	jsr iec_tx_byte		; XXX what about the status of this operation?
-	;; Mark the buffer as invalid
-	lda #$00
-	sta C3PO
 *
-	;; Buffer empty send the command
+	;; Buffer empty, send the command
 	lda #$3F
+	sta BSOUR
 	jmp iec_tx_command