more work towards IEC bus. Also don't corrupt $DD00 VIC-II bank bits in the process.

c64/kernal/iec_assert_clk_and_data.s

@@ -1,6 +1,6 @@
 iec_assert_clk_and_data:
 	lda $dd00
-	and #$08
+	and #$0b
 	ora #$30
 	sta $dd00
 	rts

c64/kernal/iec_assert_clk_release_data.s

@@ -1,6 +1,6 @@
 iec_assert_clk_release_data:
 	lda $dd00
-	and #$08
+	and #$0b
 	ora #$10
 	sta $dd00
 	rts

c64/kernal/iec_release_clk.s

@@ -1,5 +1,5 @@
 iec_release_clk:
 	lda $dd00
-	and #$18
+	and #$1b
 	sta $dd00
 	rts

c64/kernal/iec_release_clk_and_data.s

@@ -1,6 +1,6 @@
 iec_release_clk_and_data:	
 
 	lda $DD00
-	and #$08
+	and #$0b
 	sta $DD00
 	rts

c64/kernal/iec_tx_byte.s

@@ -1,8 +1,44 @@
 iec_tx_byte:	
 
+	pha
+	
+	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
+	pla
+	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
+
+	
 	;; Do the actual transmission of 8 bits of data
 	ldx #8
 
+	pla
+	
 iec_tx_byte_nextbit:	
 	;; Is next bit 0 or 1?
 	lsr
@@ -40,7 +76,6 @@ iec_tx_byte_l1:
 	bne -
 
 	;; Timeout - DEVICE NOT PRESENT
-	cli
 	jmp kernalerror_DEVICE_NOT_FOUND
 *
 	;; All done.

c64/kernal/open.s

@@ -11,38 +11,19 @@ open:
 
 	;; 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
+	lda #$28
 	jsr iec_tx_byte
-	
+	bcs open_error
+
+	;; Indicate success
+	lda #$00
+	clc	
+
+open_error:	
+	;; Re-enable interrupts and return
+	;; (iec_tx_byte will have set/cleared C flag and put result code
+	;; in A if it was an error).
+	cli
 	rts

c64/kernal/reset_entry.s

@@ -29,6 +29,8 @@ reset_entry:
 
 	; What do we do when finished?  A C64 jumps into the BASIC ROM
 	cli 			; Allow interrupts to happen
+
+	jsr open
 	
 	; c64 prg p269
 	jmp ($a000)