diff --git a/hw/fpga/addr_decode.sv b/hw/fpga/addr_decode.sv
index 2d012d0..32e1aa3 100644
--- a/hw/fpga/addr_decode.sv
+++ b/hw/fpga/addr_decode.sv
@@ -11,12 +11,12 @@ module addr_decode(
 );
 
 assign rom_cs = addr >= 16'h8000;
-assign sdram_cs = addr < 16'h7ff0;
+assign sdram_cs = addr < 16'h7fe0;
+assign mm_cs1 = addr >= 16'h7fe0 && addr < 16'h7ff0;
 assign hex_cs = addr >= 16'h7ff0 && addr < 16'h7ff4;
 assign uart_cs = addr >= 16'h7ff4 && addr < 16'h7ff6;
 assign board_io_cs = addr == 16'h7ff6;
 assign mm_cs2 = addr == 16'h7ff7;
-assign mm_cs1 = addr >= 16'h7ff8 && addr < 16'h7ffc;
 assign irq_cs  = addr == 16'h7fff;
 
 endmodule
diff --git a/sw/io.inc65 b/sw/io.inc65
index 61b079b..5e87bc2 100644
--- a/sw/io.inc65
+++ b/sw/io.inc65
@@ -8,4 +8,7 @@ UART_STATUS = UART + 1
 LED         = $7ff6
 SW          = LED
 
+MM_CTRL		= $7ff7
+MM_DATA		= $7fe0
+
 IRQ_STATUS  = $7fff
diff --git a/sw/mapper.h b/sw/mapper.h
new file mode 100644
index 0000000..8276b60
--- /dev/null
+++ b/sw/mapper.h
@@ -0,0 +1,12 @@
+#ifndef _MAPPER_H
+#define _MAPPER_H
+
+#include <stdint.h>
+
+void mapper_enable(uint8_t en);
+
+uint8_t mapper_read(uint8_t addr);
+void mapper_write(uint8_t data, uint8_t addr);
+
+#endif
+
diff --git a/sw/mapper.s b/sw/mapper.s
new file mode 100644
index 0000000..2ed1bc0
--- /dev/null
+++ b/sw/mapper.s
@@ -0,0 +1,32 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _mapper_enable
+.export _mapper_read, _mapper_write
+
+.autoimport on
+
+.code
+
+
+; void mapper_enable(uint8_t en)
+_mapper_enable:
+	sta MM_CTRL
+	rts
+
+_mapper_read:
+	phx
+	tax
+	lda MM_DATA,x
+	ldx #$00
+	rts
+
+_mapper_write:
+	phx
+	tax
+	jsr popa
+	sta MM_DATA,x
+	plx
+	rts
+