work towards chrin returning a line of input

c64/kernal/,aliases.s

@@ -28,12 +28,14 @@
 	.alias last_key_matrix_position $C5
 	.alias keys_in_key_buffer $C6
 	.alias reverse_video_flag $C7
+	.alias end_of_input_line $C8
+	.alias start_of_keyboard_input $C9
 	.alias current_key_index_entry $CB
 	.alias cursor_blink_disable $CC
 	.alias cursor_blink_countdown $CD
 	.alias cursor_saved_character $CE
 	.alias cursor_is_visible $CF
-	.alias input_source $D0
+	.alias keyboard_input_ready $D0
 	.alias current_screen_line_ptr $D1
 	.alias current_screen_x $D3
 	.alias quote_mode_flag $D4

c64/kernal/chrin.s

@@ -4,10 +4,89 @@
 	;; 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 - Not yet implemented, so infinite loop for now
+
+	;; 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.
+	;; XXX - Implement
+
+	;; Do we have a line of input waiting?
+	lda keyboard_input_ready
+	beq read_from_keyboard
+
+	;; Yes, we have input waiting at (start_of_keyboard_input)+keyboard_input_ready
+	;; When keyboard_input_ready = end_of_input_line, then we return a carriage return
+	;; and clear the flag
+	cmp end_of_input_line
+	bne not_end_of_input
+
+	;; Return carriage return and clear pending input flag
+	lda #$00
+	sta keyboard_input_ready
+	lda #$0d
+	sec
+	rts
+
+not_end_of_input:
+	;; Return next byte of waiting input and advance index
+	tay
+	lda (start_of_keyboard_input),y
+	inc keyboard_input_ready
+	sec
+	rts
+	
+
+read_from_keyboard:	
+
+	;; Wait for a key
 	lda keys_in_key_buffer
 	beq chrin
 
+	lda keyboard_buffer
+	cmp #$0d
+	bne not_enter
+	
+	;; It was enter.
+	;; Note that we have a line of input to return, and return the first byte thereof
+	;; after computing and storing its length.
+	;; (Compute's Mapping the 64, p96)
+
+	;; Set pointer to line of input
+	lda current_screen_line_ptr+0
+	sta start_of_keyboard_input+0
+	lda current_screen_line_ptr+1
+	sta start_of_keyboard_input+1
+
+	;; Calculate length
+	jsr get_current_line_logical_length
+	tay
+	iny
+*	dey
+	beq empty_line
+	lda (current_screen_line_ptr),y
+	cmp #$20
+	bne -
+	sty keyboard_input_ready
+	;; Print the carriage return
+	lda #$0d
+	jsr chrout
+	;; Return first char of line
+	ldy #$00
+	lda (current_screen_line_ptr),y
+	inc $d020
+	sec
+	rts
+
+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)
+	lda #$0d
+	jsr chrout
+	lda #$0d
+	sec
+	rts
+	
+not_enter:	
 	;; Print character
 	lda keyboard_buffer
 	jsr chrout