added turnaround to talker to IEC support

c64/kernal/iec_turnaround_to_talk.s

@@ -4,9 +4,15 @@
 
 iec_turnaround_to_talk:
 
-	;; XXX implement this
-	
+	;; First make sure both CLK, DATA and ATN are released at our side
+	jsr iec_release_atn
+	jsr iec_release_clk_data
+
+	;; Wait till device releases CLK line
+	jsr iec_wait_for_clk_release
+
+	;; Pull CLK line, it should be held by talker
+	jsr iec_pull_clk_release_data
+
 	clc
 	rts
-
-

c64/kernal/iec_wait_for_data_pull.s

@@ -0,0 +1,7 @@
+
+iec_wait_for_data_pull:
+	lda CI2PRA
+	;; Check the highest bit, which is DATA IN,
+	;; (highest bit set = negative value)
+	bmi iec_wait_for_data_pull
+	rts

c64/kernal/load.s

@@ -198,20 +198,18 @@ load_done:
 	lda #$0D
 	jsr JCHROUT
 *
-	;; Close file on drive - start by turnaround to talker
-	jsr iec_turnaround_to_talk
-	
+	;; Close file on drive
+
 	;; Command drive to stop talking and to close the file
 	jsr untlk
-	
-	;; XXX XXX code below runs into infinite loop - so for now just quit
-	clc
-	cli
-	rts
-	
+
+	;; Turnaround to talker - according to https://www.pagetable.com/?p=1135
+	;; the turnaround only happens after a command
+	jsr iec_turnaround_to_talk
+
 	lda current_device_number
 	jsr listen
-	
+
 	lda #$e0
 	jsr iec_tx_command
 
@@ -227,7 +225,7 @@ load_done:
 	clc
 	cli
 	rts
-	
+
 load_error:
 
 	;; Advance cursor to next line, if needed
@@ -239,7 +237,7 @@ load_error:
 	;; XXX - Indicate KERNAL (not BASIC) LOAD error condtion
 	sec
 	lda #28
-	
+
 	;; Re-enable interrupts and return
 	cli
 	;; (iec_tx_byte will have set/cleared C flag and put result code