started working on autostart software support

c64/kernal/f6ed.stop.s

@@ -0,0 +1,29 @@
+
+;;
+;; Official Kernal routine, described in:
+;;
+;; - [RG64] C64 Programmer's Reference Guide   - page 301
+;; - [CM64] Compute's Mapping the Commodore 64 - page 232
+;;
+;; CPU registers that has to be preserved (see [RG64]): .Y
+;;
+
+stop:
+	;; Bit 7 of STKEY contains the state of the STOP key
+	;; (Compute's Mapping the 64, p27)
+
+	;; BASIC checks carry flag to indicate STOP or not
+	lda STKEY
+	and #$80
+	beq stop_pressed
+
+	;; By trial and error, we know that Z + C = BREAK
+	;; and that neither should be set otherwise
+	lda #$ff
+	clc
+	rts
+
+stop_pressed:
+	sec
+	lda #$00
+	rts