documentation update, added STATUS.md document

c64/kernal/readst.s

@@ -1,10 +1,18 @@
-; Function defined on pp272-273 of C64 Programmers Reference Guide
+
+;;
+;; Official Kernal routine, described in:
+;;
+;; - [RG64] C64 Programmer's Reference Guide   - page 292
+;; - [CM64] Compute's Mapping the Commodore 64 - page 239
+;;
+;; CPU registers that has to be preserved (see [RG64]): .X, .Y
+;;
 
 readst:
 
 	;; Check the current device number
 
-	lda DFLTN
+	lda current_device_number
 	cmp #$02
 	bne +