added proper buffer handling to several IEC commands

c64/kernal/talk.s

@@ -5,6 +5,7 @@
 ;; - [RG64] C64 Programmer's Reference Guide   - page 301/302
 ;; - [CM64] Compute's Mapping the Commodore 64 - page 223
 ;; - 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
 ;;
@@ -12,10 +13,19 @@
 
 talk:
 
-	;; First check whether device number is correct
+	;; According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
+	jsr iec_tx_flush
+
+	;; Check whether device number is correct
 	jsr iec_devnum_check
 	bcs kernalerror_DEVICE_NOT_FOUND
+
 	;; Encode and execute the command
 	ora #$40
+
+common_talk_listen: ; common part of TALK and LISTEN
+
 	sta BSOUR
 	jmp iec_tx_command
+
+