begin implementing IEC serial protocol.

c64/kernal/iec_tx_byte.s

@@ -0,0 +1,50 @@
+iec_tx_byte:	
+
+	;; Do the actual transmission of 8 bits of data
+	ldx #8
+
+iec_tx_byte_nextbit:	
+	;; Is next bit 0 or 1?
+	lsr
+	pha
+	bcs +
+	
+	;; Setup 0
+	jsr iec_assert_clk_release_data
+	jmp iec_tx_byte_l1
+*
+	;; Setup 1
+	jsr iec_assert_clk_and_data
+iec_tx_byte_l1:
+	;; Clock bit out
+	jsr iec_wait20us
+	jsr iec_release_clk
+	jsr iec_wait20us
+
+	;; More bits to send?
+	pla
+	dex
+	bpl iec_tx_byte_nextbit
+
+	;; Done sending bits. Wait for acknowledgement
+	jsr iec_assert_clk_release_data
+
+	;; Acknowledgement must happen within 1ms, else it
+	;; is DEVICE NOT PRESENT
+	;; We can't easily count rasters here, so we just
+	;; try a set number of times
+	ldx #$ff
+*	lda $DD00
+	bmi +
+	dex
+	bne -
+
+	;; Timeout - DEVICE NOT PRESENT
+	cli
+	jmp kernalerror_DEVICE_NOT_FOUND
+*
+	;; All done.
+	;; End state is TALKER is asserting CLK, and listener is asserting DATA
+	;; until they are ready for the next byte.
+	clc
+	rts