Add null mapper

hw/efinix_fpga/mapper.sv

@@ -0,0 +1,38 @@
+module mapper(
+    input clk,
+    input rst,
+
+    input [15:0] cpu_addr,
+    output logic [24:0] sdram_addr,
+
+    input cs,
+    input rw,
+
+    input [7:0] i_data,
+    output logic [7:0] o_data
+);
+
+logic [12:0] map [16];
+
+logic [15:0] base_addr;
+
+assign base_addr = cpu_addr - 16'hefb7;
+
+logic en;
+
+always_comb begin
+    if (!en) begin
+        sdram_addr = {9'b0, cpu_addr};
+    end
+end
+
+always_ff @(posedge clk) begin
+    if (rst) begin
+        en <= '0;
+    end
+end
+
+// each each entry is 4k and total address space is 64M,
+// so we need 2^14 possible entries
+
+endmodule