added full protection against illegal IEC device numbers

c64/kernal/iec_devnum_check.s

@@ -0,0 +1,24 @@
+
+;;
+;; Checks whether device numbe (in .A) is a correct IEC device number
+;;
+;; Preserves .A, .X and .Y
+;;
+
+iec_devnum_check:
+	;; 0, 1, 2, or 3 are illegal - reserved for non-IEC devices on Commodore
+	cmp #$03
+	bpl +
+	bcs +
+	;; FALLTROUGH
+iec_devnum_check_failed:
+	sec
+	rts
+*
+	;; 30 and above are illegal too, due to IEC command encoding scheme,
+	;; see https://www.pagetable.com/?p=1031
+	cmp #$1E
+	bpl iec_devnum_check_failed
+	bcs iec_devnum_check_failed
+	clc
+	rts