Starting work on a simple cartridge

In order to get through bootstrap process

rtl/gb.sv

@@ -1,16 +1,16 @@
 // Top level gb module
 module gb (
-  input  logic clk,
-  input  logic nreset,
+  input  wire clk,
+  input  wire nreset,
 
   // Cartridge bus
-  output  logic        cart_clk_o,
-  output  logic        cart_nreset_o,
-  output  logic        cart_nrd_o,
-  output  logic        cart_nwr_o,
-  output  logic        cart_ncs_o,
-  output  logic [15:0] cart_addr_o,
-  input   logic [ 7:0] cart_data_i
+  output  wire        cart_clk_o,
+  output  wire        cart_nreset_o,
+  output  wire        cart_nrd_o,
+  output  wire        cart_nwr_o,
+  output  wire        cart_ncs_o,
+  output  wire [15:0] cart_addr_o,
+  inout   wire [ 7:0] cart_data_io
 
 );
 
@@ -31,6 +31,9 @@ logic [ 7:0] hiram_rdata;
 logic        vram_sel;
 logic [ 7:0] vram_rdata;
 
+logic        cart_sel;
+logic [ 7:0] cart_rdata;
+
 cpu cpu_inst (
   .clk    (clk),
   .nreset (nreset),
@@ -56,11 +59,13 @@ assign rom_enable_r = '1;
 assign rom_sel   = rom_enable_r & ~(|cpu_addr[15:8]);
 assign vram_sel  = (cpu_addr[15:13] == 3'b100);
 assign hiram_sel = (&cpu_addr[15:7]) & ~(&cpu_addr[6:0]);
+assign cart_sel  = (~cpu_addr[15]) & (~cpu_addr[14]) & (~rom_sel);
 
 assign cpu_rdata = rom_rdata     |
                    vram_rdata    |
                    hiram_rdata   |
-                   cpu_ppu_rdata;
+                   cpu_ppu_rdata |
+                   cart_rdata;
 
 rom #(
   .FILE_NAME("DMG_ROM.bin"),
@@ -100,6 +105,13 @@ ram #(
   .wdata_i   (cpu_wdata)
 );
 
+assign cart_rdata = cart_sel ? cart_data_io : '0;
 
+assign cart_clk_o    = clk;
+assign cart_nreset_o = nreset;
+assign cart_nrd_o    = 1'b0;
+assign cart_nwr_o    = 1'b1;
+assign cart_ncs_o    = ~cart_sel;
+assign cart_addr_o   = cpu_addr;
 
 endmodule : gb