added proper buffer handling to several IEC commands

c64/kernal/unlsn.s

@@ -5,6 +5,7 @@
 ;; - [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
+;; - http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
 ;;
 ;; CPU registers that has to be preserved (see [RG64]): .X, .Y
 ;;
@@ -12,19 +13,19 @@
 
 unlsn:
 
-	;; This routine is documented in 'Compute's Mapping the Commodore 64', page 224
-	
-	;; First check whether we have something to be sent in the buffer
-	lda C3PO
-	beq +
-	sec ; send it with EOI
-	jsr iec_tx_byte ; send the command regardless of the status
-*
+	;; According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
+	jsr iec_tx_flush
+
 	;; Buffer empty, send the command
 	lda #$3F
+
+common_unlsn_second: ; common part of UNSLS and SECOND
+
 	sta BSOUR
 	jsr iec_tx_command
 	bcs + ; branch if error
 	jmp iec_tx_command_finalize
 *
 	rts
+
+