Add init code for mapper

init_mapper now remaps so that it can change irq vectors

sw/kernel/devices/mapper.s

@@ -0,0 +1,38 @@
+.MACPACK generic
+
+.autoimport
+
+.export _init_mapper
+.export _map
+
+MAPPER_BASE = $200
+
+.code
+
+; void init_paging();
+; This should be identity mapped at reset, but we can do it again anyway
+.proc _init_mapper
+    ldx #$00
+L1:
+    txa
+    lsr
+    sta MAPPER_BASE,x
+    lda #$00
+    sta MAPPER_BASE+1,x
+    inx
+    inx
+    cpx #$20
+    blt L1
+    rts 
+.endproc
+
+; void map(uint16_t p_page, uint8_t v_page); 
+.proc _map
+    asl
+    tax                 ; x = v_page * 2
+    jsr popa            ; low byte of p_page
+    sta MAPPER_BASE,x   ; write low byte to mm_low
+    jsr popa            ; high byte of p_page
+    sta MAPPER_BASE+1,x ; write high byte to mm_high
+    rts
+.endproc