more work towards internal MEGA65 DOS

src/kernal/board_m65/mega65_map_helpers.s

@@ -6,8 +6,10 @@
 //
 
 // Available memory maps:
-// - NORMAL   - nothing mapped in
-// - KERNAL_1 - for calling KERNAL_1 segment code
+// - NORMAL            - nothing mapped in
+// - KERNAL_1          - for calling KERNAL_1 segment code
+// - DOS_1             - for swiching to DOS context
+// - NORMAL_from_DOS_1 - for switching bact from DOS context
 
 
 map_NORMAL:
@@ -19,6 +21,11 @@ map_NORMAL:
 	phz
 
 	lda #$00
+
+	// FALLTROUGH
+
+map_NORMAL_common:
+
 	tax
 	tay
 	taz
@@ -30,6 +37,10 @@ map_end:
 	map
 	eom
 
+	// FALLTROUGH
+
+map_end_no_eom:
+
 	plz
 	ply
 	plx
@@ -53,4 +64,37 @@ map_KERNAL_1:
 
 	ldx #$42    // 0x4000 <- map 8KB from 0x20000
 
-	bne map_end
+	jmp_8 map_end
+
+map_DOS_1:
+
+	php
+	pha
+	phx
+	phy
+	phz
+
+	lda #$C0
+	tab                      // from now on, zeropage starts from $C000
+
+	ldy #$00
+	taz #$81                 // 0xC000 <- map 8KB (4KB useable) from $10000
+
+	tya
+	ldx #$C2                 // 0x4000 <- map 16KB from 0x20000
+
+	map
+	jmp_8 map_end_no_eom     // no EOM, we do not want interrupts within DOS!
+
+map_NORMAL_from_DOS_1:
+
+	php
+	pha
+	phx
+	phy
+	phz
+
+	lda #$00
+	tba
+
+	jmp_8 map_NORMAL_common