several IEC routines now properly preserve X/Y registers

c64/kernal/iec_return.s

@@ -0,0 +1,38 @@
+
+;;
+;; Helper routines to return with error/success
+;;
+;; All of them restore X/Y registers from stack,
+;; they take care about Carry flag,
+;; error return reeset IEC line to default state,
+;; take care about .A and IOSTATUS
+;;
+
+iec_return_success:
+
+	;; BSOUR is certainly not valid anymore
+	lda #$00
+	sta C3PO
+	;; Restore registers
+	pla
+	tay
+	pla
+	tax
+	;; Report success
+	clc
+	rts
+
+iec_return_DEVICE_NOT_FOUND:
+
+	;; Set IEC line to normal idle state
+	jsr iec_set_idle
+	;; BSOUR is certainly not valid anymore
+	lda #$00
+	sta C3PO
+	;; Restore registers
+	pla
+	tay
+	pla
+	tax
+	;; Report error
+	jmp kernalerror_DEVICE_NOT_FOUND