From 841081c15207c6de375cdb2236bbabf7ad9fb145 Mon Sep 17 00:00:00 2001
From: jl7719 <jl7719@ic.ac.uk>
Date: Sun, 29 Nov 2020 17:44:08 +0900
Subject: [PATCH] Update mips_cpu_memory.v

Change as per constraint
---
 rtl/mips_cpu_memory.v | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/rtl/mips_cpu_memory.v b/rtl/mips_cpu_memory.v
index 631d766..e13b879 100644
--- a/rtl/mips_cpu_memory.v
+++ b/rtl/mips_cpu_memory.v
@@ -5,6 +5,13 @@ Memory for Harvard Interface
 - combinatorial read/fetch of instruction via instr_ port
 - combinatorial read and single cycle write of data via data_ port
 
+Constraints
+read write
+  0    0   -> nothing
+  0    1   -> write
+  1    0   -> read
+  1    1   -> read ????? probs should never occur?
+
 Instantiation of Memory Module
 - mips_cpu_memory #(RAM_INIT_FILE) ramInst(clk, data_address, data_write, data_read, data_writedata, data_readdata, instr_address, instr_readdata);
 
@@ -15,11 +22,12 @@ Special Memory Locations
 
 Needs checking with:
 -- clk or clk_enable?
+-- constraint on not being able to read and write in the same cycle
 -- whether there is a more efficient way of initialising memory to zero (line 32)
 */
 
 module mips_cpu_memory(
-    input logic clk,
+    input logic clk_enable,
 
     //Data Memory
     input logic[31:0] data_address,
@@ -51,14 +59,24 @@ module mips_cpu_memory(
     end
 
     //Combinatorial read path for data and instruction.
-    assign data_readdata = data_read ? memory[data_address] : 16'hxxxx;
-    assign instr_readdata = memory[instr_address]
+    if (clk_enable == 1) begin
+        assign data_readdata = data_read ? memory[data_address] : 16'hxxxx;
+        assign instr_readdata = memory[instr_address];
+    end
+    else begin
+        assign data_readdata = data_readdata;
+        assign instr_readdata = instr_address;
+    end
+
 
     //Synchronous write path
-    always_ff @(posedge clk) begin
-        //$display("RAM : INFO : data_read=%h, data_addr = %h, mem=%h", data_read, data_address, memory[data_address]);
-        if (!data_read & data_write) begin  //cannot read and write to memory in the same cycle
-            memory[data_address] <= data_writedata;
+    always_ff @(posedge clk_enable) begin
+        //$display("RAM : INFO : data_read=%h, data_address = %h, mem=%h", data_read, data_address, memory[data_address]);
+        if (!data_read & data_write) begin              //cannot read and write to memory in the same cycle
+            if (instr_address != data_address) begin    //cannot modify the instruction being read
+                memory[data_address] <= data_writedata;            
+            end
         end
     end
-endmodule
\ No newline at end of file
+endmodule
+