implemented READST routine

c64/kernal/readst.s

@@ -1,3 +1,22 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
+
 readst:
+
+	;; Check the current device number
+
+	lda DFLTN
+	cmp #$02
+	bne +
+	
+	;; This is RS-232 device - according to 'Compute's Mapping the Commodore 64' page 239
+	;; it reads status from RSSTAT
+	lda RSSTAT
+	rts
+*
+	;; According to 'Compute's Mapping the Commodore 64' page 239, it usually retrieves
+	;; status from IOSTATUS
+	
+	;; XXX is it always the case if device is not RS-232 ?
+
+	lda IOSTATUS
 	rts