Hazard unit and tests

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diff --git a/rv64cpu/rtl/core/hazard_unit.sv b/rv64cpu/rtl/core/hazard_unit.sv
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+//     Richard CPU
+//     Copyright (C) 2026  Richard Qin
+
+//     This program is free software: you can redistribute it and/or modify
+//     it under the terms of the GNU General Public License as published by
+//     the Free Software Foundation, either version 3 of the License, or
+//     (at your option) any later version.
+
+//     This program is distributed in the hope that it will be useful,
+//     but WITHOUT ANY WARRANTY; without even the implied warranty of
+//     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//     GNU General Public License for more details.
+
+//     You should have received a copy of the GNU General Public License
+//     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+// File path: rtl/core/hazard_unit.sv
+// Description: Hazard Control Unit
+// Handles data hazards (forwarding, Load-Use stalls), control hazards
+// (branch misprediction flushes), and memory stalls (D-Cache misses).
+
+module hazard_unit
+    import rv64_pkg::*;
+(
+    // -----------------------
+    // Inputs from ID Stage (for Load-Use detection)
+    // -----------------------
+    input  logic [4:0]  id_rs1_addr,
+    input  logic [4:0]  id_rs2_addr,
+
+    // -----------------------
+    // Inputs from EX Stage (ID/EX register outputs)
+    // -----------------------
+    input  logic [4:0]  ex_rs1_addr,
+    input  logic [4:0]  ex_rs2_addr,
+    input  logic [4:0]  ex_rd_addr,
+    input  logic        ex_reg_write_en,
+    input  logic        ex_is_load,
+    input  logic        ex_branch_taken,
+
+    // -----------------------
+    // Inputs from MEM Stage (EX/MEM register outputs)
+    // -----------------------
+    input  logic [4:0]  mem_rd_addr,
+    input  logic        mem_reg_write_en,
+    input  logic        mem_stall_req,       // D-Cache miss or memory busy
+
+    // -----------------------
+    // Inputs from WB Stage (MEM/WB register outputs)
+    // -----------------------
+    input  logic [4:0]  wb_rd_addr,
+    input  logic        wb_reg_write_en,
+
+    // -----------------------
+    // Outputs: EX-stage operand forwarding mux selects
+    // -----------------------
+    // 00: no forward (use regfile), 01: forward from MEM, 10: forward from WB
+    output logic [1:0]  forward_a_sel,
+    output logic [1:0]  forward_b_sel,
+
+    // -----------------------
+    // Outputs: Pipeline stall and flush controls
+    // -----------------------
+    output logic        if_stall_en,
+    output logic        if_id_stall_en,
+    output logic        id_ex_stall_en,
+    output logic        ex_mem_stall_en,
+    output logic        if_id_flush_en,
+    output logic        id_ex_flush_en
+);
+
+    // -----------------------
+    // Data Forwarding (EX-stage operand bypass)
+    // -----------------------
+    // Forwarding resolves RAW hazards by routing results from
+    // MEM or WB back to the EX ALU inputs. We compare EX-stage
+    // source register addresses (the instruction currently in EX)
+    // against the destination registers of instructions in MEM/WB.
+    //
+    // Priority: MEM > WB (MEM result is more recent).
+
+    // Operand A (ex_rs1_addr)
+    always_comb begin
+        if (mem_reg_write_en && (mem_rd_addr != 5'b0) && (mem_rd_addr == ex_rs1_addr)) begin
+            forward_a_sel = 2'b01; // Forward from MEM stage
+        end else if (wb_reg_write_en && (wb_rd_addr != 5'b0) && (wb_rd_addr == ex_rs1_addr)) begin
+            forward_a_sel = 2'b10; // Forward from WB stage
+        end else begin
+            forward_a_sel = 2'b00; // No forwarding
+        end
+    end
+
+    // Operand B (ex_rs2_addr)
+    always_comb begin
+        if (mem_reg_write_en && (mem_rd_addr != 5'b0) && (mem_rd_addr == ex_rs2_addr)) begin
+            forward_b_sel = 2'b01; // Forward from MEM stage
+        end else if (wb_reg_write_en && (wb_rd_addr != 5'b0) && (wb_rd_addr == ex_rs2_addr)) begin
+            forward_b_sel = 2'b10; // Forward from WB stage
+        end else begin
+            forward_b_sel = 2'b00; // No forwarding
+        end
+    end
+
+    // -----------------------
+    // Load-Use Hazard Detection
+    // -----------------------
+    // If the instruction in EX is a load, and the instruction in
+    // ID reads from the load's destination, we must stall for 1
+    // cycle because the load data is not available until MEM.
+    logic load_use_stall;
+    assign load_use_stall = ex_is_load && (ex_rd_addr != 5'b0) &&
+                            ((ex_rd_addr == id_rs1_addr) || (ex_rd_addr == id_rs2_addr));
+
+    // -----------------------
+    // Stall and Flush Signal Generation
+    // -----------------------
+    always_comb begin
+        // Defaults: no stalls, no flushes
+        if_stall_en    = 1'b0;
+        if_id_stall_en = 1'b0;
+        id_ex_stall_en = 1'b0;
+        ex_mem_stall_en = 1'b0;
+        if_id_flush_en = 1'b0;
+        id_ex_flush_en = 1'b0;
+
+        // (1) Memory stall (D-Cache miss): freeze the entire pipeline
+        if (mem_stall_req) begin
+            if_stall_en    = 1'b1;
+            if_id_stall_en = 1'b1;
+            id_ex_stall_en = 1'b1;
+            ex_mem_stall_en = 1'b1;
+        end
+
+        // (2) Load-Use stall: freeze IF and IF/ID, insert bubble into ID/EX
+        if (load_use_stall && !mem_stall_req) begin
+            if_stall_en    = 1'b1;
+            if_id_stall_en = 1'b1;
+            id_ex_flush_en = 1'b1;
+        end
+
+        // (3) Branch/jump misprediction: flush IF/ID and ID/EX
+        // Branch flush overrides load-use stall (the stalled instruction is invalid anyway)
+        if (ex_branch_taken) begin
+            if_id_flush_en = 1'b1;
+            id_ex_flush_en = 1'b1;
+            // Cancel load-use stall if branch is also taken
+            if_stall_en    = 1'b0;
+            if_id_stall_en = 1'b0;
+        end
+    end
+
+endmodule