fix preserving X register in CHRIN

c64/kernal/chrin.s

@@ -4,9 +4,11 @@
 	;; If from keyboard, then it gets a whole line of input, and returns the first char.
 	;; Repeated calls after that read out the successive bytes of the line of input.
 chrin:
-	;; XXX - Don't corrupt X
-	stx last_printed_character_ascii
-	
+	;; Save X
+	txa
+	pha
+
+chrin_repeat:	
 	;; Do we have a line of input we are currently returning?
 	;; If so, return the next byte, and clear the flag when we reach the end.
 
@@ -23,7 +25,8 @@ chrin:
 	;; Return carriage return and clear pending input flag
 	lda #$00
 	sta keyboard_input_ready
-	ldx last_printed_character_ascii
+	pla
+	tax
 	lda #$0d
 	clc
 	rts
@@ -31,7 +34,8 @@ chrin:
 not_end_of_input:
 	;; Return next byte of waiting input and advance index
 	tay
-	ldx last_printed_character_ascii
+	pla
+	tax
 	lda (start_of_keyboard_input),y
 	inc keyboard_input_ready
 	clc
@@ -42,7 +46,7 @@ read_from_keyboard:
 
 	;; Wait for a key
 	lda keys_in_key_buffer
-	beq chrin
+	beq chrin_repeat
 
 	lda keyboard_buffer
 	cmp #$0d
@@ -78,7 +82,8 @@ read_from_keyboard:
 	sta keyboard_input_ready
 	;; Return first char of line
 	ldy #$00
-	ldx last_printed_character_ascii
+	pla
+	tax
 	lda (start_of_keyboard_input),y
 	clc
 	rts
@@ -87,7 +92,8 @@ empty_line:
 	;; For an empty line, just return the carriage return
 	;; (and don't forget to actually print the carriage return, so that
 	;; the cursor advances and screen scrolls as required)
-	ldx last_printed_character_ascii
+	pla
+	tax
 	lda #$0d
 	clc
 	rts
@@ -100,7 +106,7 @@ not_enter:
 	jsr pop_keyboard_buffer
 
 	;; Keep looking for input from keyboard until carriage return
-	jmp chrin
+	jmp chrin_repeat
 
 pop_keyboard_buffer:	
 	;; Pop key out of keyboard buffer