rework in CBDOS

src/dos_m65/dos_enter_exit.s

@@ -0,0 +1,54 @@
+
+; Helper routines for entering/exiting the context
+
+
+
+dos_ENTER:         ; enter DOS context
+ 
+	sta REG_A
+	stx REG_X
+	sty REG_Y
+	stz REG_Z
+
+	rts
+
+
+dos_EXIT_SEC:      ; exit DOS context with Carry set
+
+	sec
+	bra dos_EXIT_common
+
+dos_EXIT:          ; exit DOS context with Carry clear
+
+	clc
+
+	; FALLTROUGH
+
+dos_EXIT_common:
+
+	ldz REG_Z
+	ldy REG_Y
+	ldx REG_X
+	lda REG_A
+
+	rts
+
+
+dos_EXIT_A_SEC:    ; exit DOS context with Carry set, return .A
+
+	sec
+	bra dos_EXIT_A_common
+
+dos_EXIT_A:        ; exit DOS context with Carry clear, return .A
+
+	clc
+
+	; FALLTROUGH
+
+dos_EXIT_A_common:
+
+	ldz REG_Z
+	ldy REG_Y
+	ldx REG_X
+
+	rts