From 714b74ec833cce7f381b84471e4a6a0347f415df Mon Sep 17 00:00:00 2001
From: Aadi Desai <21363892+supleed2@users.noreply.github.com>
Date: Fri, 11 Dec 2020 19:13:11 +0000
Subject: [PATCH] Update mips_cpu_bus.v

Added fetch/execute states. All instructions not using data memory should function
---
 rtl/mips_cpu_bus.v | 86 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 79 insertions(+), 7 deletions(-)

diff --git a/rtl/mips_cpu_bus.v b/rtl/mips_cpu_bus.v
index f43174d..a2e0792 100644
--- a/rtl/mips_cpu_bus.v
+++ b/rtl/mips_cpu_bus.v
@@ -15,18 +15,90 @@ module mips_cpu_bus(
     input logic[31:0] readdata
 );
 
+logic[1:0] state; // current state of cpu within cycle
+logic[1:0] n_state; // state to be set at next clk edge
 logic[31:0] instr_reg; // instruction register / single-word cache for current instruction
 logic clk_internal; // modulated clock to be passed to harvard cpu
+logic[31:0] harvard_instr_address; // instr addr from pc
+logic harvard_read; // harvard cpu read flag
+logic harvard_write; // harvard cpu write flag
 
-always_ff @(posedge clk) begin // how/when to pass through negedge?
-    if (waitrequest) begin //if waitrequest is high, do nothing
+initial begin
+    clk_internal = 1'b0;
+    n_state = 2'b00;
+    state = 2'b00;
+    instr_reg = 32'h00000000;
+    address = 32'h00000000;
+    write = 1'b0;
+    read = 1'b0;
+    writedata = 32'h00000000;
+    byteenable = 4'b0000;
+end
+
+always_ff @(posedge clk) begin // CLK Rising Edge
+    if (waitrequest) begin
     end else begin
-        //update outputs?
+        case (n_state)
+            2'b00: begin // fetch
+                clk_internal <= 1'b1;
+                state <= 2'b00;
+            end
+            2'b01: begin // execute
+                state <= 2'b10;
+                instr_reg <= readdata;
+            end
+            2'b10: begin // read
+            end
+            2'b11: begin // write
+            end
+        endcase // state
     end
 end
 
-always_comb begin
+always_ff @(negedge clk) begin // CLK Falling Edge
+    case (state)
+        2'b00: // nothing happens on fetch negedge
+        2'b01: begin // execute negedge
+            if (!harvard_read && !harvard_write) begin // instruction complete, trigger writeback
+                clk_internal <= 1'b0;
+            end // otherwise do nothing
+        end
+    endcase
+end
 
+always_comb begin
+    if (reset) begin
+        clk_internal = 1'b0;
+        n_state = 2'b00;
+        state = 2'b00;
+        instr_reg = 32'h00000000;
+        address = 32'h00000000;
+        write = 1'b0;
+        read = 1'b0;
+        writedata = 32'h00000000;
+        byteenable = 4'b0000;
+    end else begin
+        case (state)
+            2'b00: begin
+                address = harvard_instr_address;
+                read = 1'b1;
+                byteenable = 4'b1111;
+                n_state = 2'b01;
+            end
+            2'b01: begin
+                address = 32'h00000000;
+                read = 1'b0;
+                byteenable = 4'b0000;
+                if (harvard_read) begin
+                    n_state = 2'b10; // next state is read
+                end else if (harvard_write) begin
+                    n_state = 2'b11; // next state is write
+                end else begin
+                    n_state 2'b00; // next state is fetch
+                end
+            end
+        endcase // state
+    end
 end
 
 mips_cpu_harvard mips_cpu_harvard( // Harvard CPU within wrapper
@@ -35,11 +107,11 @@ mips_cpu_harvard mips_cpu_harvard( // Harvard CPU within wrapper
 .active(active), // Is CPU active, output
 .register_v0(register_v0), // $2 / $v0 debug bus, output
 .clk_enable(1'b0), // unused clock enable, input
-.instr_address(######), // output
+.instr_address(harvard_instr_address), // instr addr from pc, output
 .instr_readdata(instr_reg), // cached instruction passed into harvard cpu, input
 .data_address(######), // output
-.data_write(######), // output
-.data_read(######), // output
+.data_write(harvard_write), // harvard write flag, output
+.data_read(harvard_read), // harvard read flag, output
 .data_writedata(######), // output
 .data_readdata(######) // input
 );