implemented READST routine

c64/basic/,aliases.s

@@ -58,7 +58,8 @@
 	.alias BUCKYSTATUS $91
 	.alias load_or_verify_flag $93
 	.alias current_file_num $98
-	.alias input_device_number $99
+	.alias DFLTN $99
+	.alias input_device_number DFLTN
 	; bit 6 = error messages, bit 7 = control messages
 	.alias kernal_message_control_flags $9d
 	.alias jiffy_clock_24bits $a0
@@ -131,6 +132,7 @@
 	.alias keyboard_decoding_ptr $028F
 	.alias enable_case_switch $0291
 	.alias screen_scroll_disable $0292
+	.alias RSSTAT $0297 ; RS-232 status
 	.alias pal_or_ntsc $02A6
 
 	;; Registers for SYS call

c64/kernal/,aliases.s

@@ -9,7 +9,8 @@
 	.alias BUCKYSTATUS $91
 	.alias kernal_load_or_verify_flag $93
 	.alias current_file_num $98
-	.alias input_device_number $99
+	.alias DFLTN $99
+	.alias input_device_number DFLTN
 	; bit 6 = error messages, bit 7 = control messages
 	.alias kernal_message_control_flags $9d
 	.alias jiffy_clock_24bits $a0
@@ -79,6 +80,7 @@
 	.alias keyboard_decoding_ptr $028F
 	.alias enable_case_switch $0291
 	.alias screen_scroll_disable $0292
+	.alias RSSTAT $0297 ; RS-232 status
 	.alias pal_or_ntsc $02A6
 
 	;; Kernal vectors - interrupts

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