added ZP shadowing for monitor and Z80 CPU emulator

src/kernal/board_m65/4000.mega65_vectable_kernal_1.s

@@ -26,6 +26,7 @@
 	!addr VK1__M65_CLRSCR                = $4000 + 2 * 12
 	!addr VK1__M65_CLRWIN                = $4000 + 2 * 13
 	!addr VK1__m65_screen_upd_txtrow_off = $4000 + 2 * 14
+	!addr VK1__m65_shadow_BZP            = $4000 + 2 * 15
 
 } else {
 
@@ -60,4 +61,5 @@
 	!word M65_CLRSCR	
 	!word M65_CLRWIN
 	!word m65_screen_upd_txtrow_off
+	!word m65_shadow_BZP
 }

src/kernal/board_m65/mega65_monitor.s

@@ -4,7 +4,8 @@
 
 MONITOR:
 
+	jsr m65_shadow_BZP
 	jsr map_MON_1
 	jmp ($4000)
 
-	; Monitor knows, that it should exit via map_NORMAL
+	; Monitor knows, that it should exit via m65_shadow_BZP

src/kernal/board_m65/mega65_shadow_bzp.s

@@ -0,0 +1,80 @@
+;; #LAYOUT# M65 KERNAL_0 #TAKE
+;; #LAYOUT# M65 KERNAL_1 #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+; Exchanges BASIC part of the zero page with it's shadow location; tries to destroy as little memory as possible
+
+
+m65_shadow_BZP:
+
+!ifdef SEGMENT_M65_KERNAL_0 {
+
+	jsr map_KERNAL_1
+	jsr (VK1__m65_shadow_BZP)
+	jmp map_NORMAL
+
+} else {
+
+
+	; Preserve CPU status and registers
+
+	php
+	phz
+	pha
+	phx
+
+	; We need a 4-byte pointer on the zero page; let's use the ones starting from FNLEN
+
+	ldx #$03
+@1:
+	lda FNLEN,x
+	pha
+	dex
+	bpl @1
+
+	; Store the address of the ZP shadow location
+
+	lda #MEMCONF_SHADOW_BZP_0
+	sta FNLEN+0
+	lda #MEMCONF_SHADOW_BZP_1
+	sta FNLEN+1
+	lda #MEMCONF_SHADOW_BZP_2
+	sta FNLEN+2
+	lda #MEMCONF_SHADOW_BZP_3
+	sta FNLEN+3
+
+	; Exchange the ZP content
+
+	ldz #$00
+	ldx #$00
+@2:
+	lda [FNLEN],z
+	pha
+	lda $02,x
+	sta [FNLEN],z
+	pla
+	sta $02,x
+	inz
+	inx
+	cpx #$8E
+	bne @2
+
+	; Restore FNLEN and further bytes
+
+	ldx #$00
+@3:
+	pla
+	sta FNLEN,x
+	inx
+	cpx #$04
+	bne @3
+
+	; Restore registers and quit
+
+	plx
+	pla
+	plz
+	plp
+
+	rts
+}