begin implementing IEC serial protocol.

c64/kernal/open.s

@@ -1,3 +1,48 @@
-; Function defined on pp272-273 of C64 Programmers Reference Guide
+	; Function defined on pp272-273 of C64 Programmers Reference Guide
+	;; See also http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
+
+	;; Sequence is:
+	;; 1. Call current device to attention as LISTENER = $20 + device
+	;; 2. 
 open:
+
+	;; Disable IRQs, since timing matters!
+	SEI
+
+	;; Begin sending under attention
+	jsr iec_assert_atn
+	jsr iec_assert_clk_release_data
+
+	;; Give devices time to respond
+	jsr wait1ms
+
+	;; Did a device respond? (DATA will be pulled if so)
+	lda $DD00
+	bmi +
+
+	;; No devices present, so we can immediately return with device not found
+	cli
+	jmp kernalerror_DEVICE_NOT_FOUND
+*
+
+	;; Release CLK and wait for DATA to release.
+	jsr iec_release_clk_and_data
+	
+	;; Wait for DATA to be released
+	;; Listener is allowed to hold DATA for as long as they want.
+	;; i.e., this is the part of the protocol where the listener indicates
+	;; that they are busy doing something, like processing the previous
+	;; character.
+	jsr iec_wait_for_data_release
+
+	;; We then wait for 60 usec (less than 200usec so we don't
+	;; signal EOI), before asserting CLK again.
+	jsr iec_wait60us
+
+	;; Re-assert CLK line
+	jsr iec_assert_clk_release_data
+
+	;; CLK is now asserted, and we are ready to transmit a byte
+	jsr iec_tx_byte
+	
 	rts