began work on the central routing logic, updated some documentation

fabric/src/mem_hub.sv

@@ -0,0 +1,50 @@
+module mem_hub (input logic             rst,
+                input logic             sys_clk,
+                input logic [3:0]       connected_devices, // manually configured
+                input logic [3:0][7:0]  rx_cmd,            // for routing-related commands
+                input logic [3:0]       rx_cmd_valid,
+                input logic [3:0][7:0]  rx_byte,
+                input logic [3:0]       rx_valid,
+                input logic [3:0][1:0]  rx2tx_dest,        // rx byte's destination
+                input logic [3:0]       tx_read,           // if tx_byte was read
+                output logic [3:0]      rx_read,           // if rx_byte was read
+                output logic [3:0][1:0] tx_src,            // tell the tx where the stream is comming from
+                output logic [3:0][7:0] tx_byte,
+                output logic [3:0]      tx_valid,
+                output logic [1:0]      packet_size);      // 4 states for 4 fixed packet sizes
+   timeunit 1ns;
+   timeprecision 1ps;
+   
+   // TBD: pre-agree on packet size
+
+   // [index][rx_src]
+   logic [3:0][1:0] service_queue;
+   logic [3:0]      in_queue;
+
+   // [rx_src][tx_dest], might not be useful
+   logic [1:0][1:0] rx2tx_map;
+   
+   logic [2:0]      i;
+
+   always_ff @ (posedge sys_clk) begin
+      if (rst) begin
+         rx_read <= '0;
+         tx_src <= '0;
+         tx_valid <= '0;
+         packet_size <= '0;
+         service_queue <= '0;
+         in_queue <= '0;
+         rx2tx_map <= '0;
+         i <= 0;
+      end
+      
+      if (in_queue == 4'd0) begin // no one is in the queue yet
+         if (tx_valid != 4'd0) begin
+            for (i = 0; i < 3'd4; i++) begin
+               // TODO: write the logic for enqueuing
+            end
+         end
+      end else begin
+      end
+   end
+endmodule // mem_hub