begin implementing chrin to read from keyboard buffer

c64/kernal/chrin.s

@@ -5,8 +5,27 @@
 	;; Repeated calls after that read out the successive bytes of the line of input.
 chrin:
 	;; XXX - Not yet implemented, so infinite loop for now
-	inc $d020
+	lda keys_in_key_buffer
-	jmp chrin
+	beq chrin
+
+	;; Print character
+	lda keyboard_buffer
+	jsr chrout
+
+	;; Pop key out of keyboard buffer
+	;; Disable interrupts while reading from keyboard buffer
+	;; so that no race conditions can occur
+	sei
+	ldx #$00
+	ldy #$01
+*	lda keyboard_buffer,y
+	sta keyboard_buffer,x
+	inx
+	iny
+	cpy keys_in_key_buffer
+	bne -
+	dec keys_in_key_buffer
+	cli
 	
 	lda #$00
 	clc