Subversion Repositories dbg_interface

//////////////////////////////////////////////////////////////////////

////                                                              ////

////  dbg_tb.v                                                    ////

////                                                              ////

////                                                              ////

////  This file is part of the SoC/OpenRISC Development Interface ////

////  http://www.opencores.org/projects/DebugInterface/           ////

////                                                              ////

////  Author(s):                                                  ////

////       Igor Mohor (igorm@opencores.org)                       ////

////                                                              ////

////                                                              ////

////  All additional information is avaliable in the README.txt   ////

////  file.                                                       ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

////                                                              ////

//// Copyright (C) 2000 - 2004 Authors                            ////

////                                                              ////

//// This source file may be used and distributed without         ////

//// restriction provided that this copyright statement is not    ////

//// removed from the file and that any derivative work contains  ////

//// the original copyright notice and the associated disclaimer. ////

////                                                              ////

//// This source file is free software; you can redistribute it   ////

//// and/or modify it under the terms of the GNU Lesser General   ////

//// Public License as published by the Free Software Foundation; ////

//// either version 2.1 of the License, or (at your option) any   ////

//// later version.                                               ////

////                                                              ////

//// This source 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 Lesser General Public License for more ////

//// details.                                                     ////

////                                                              ////

//// You should have received a copy of the GNU Lesser General    ////

//// Public License along with this source; if not, download it   ////

//// from http://www.opencores.org/lgpl.shtml                     ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

//

// CVS Revision History

//

// $Log: not supported by cvs2svn $

// Revision 1.34  2004/01/20 14:24:08  mohor

// Define name changed.

//

// Revision 1.33  2004/01/20 14:05:26  mohor

// Data latching changed when testing WB.

//

// Revision 1.32  2004/01/20 10:23:21  mohor

// More debug data added.

//

// Revision 1.31  2004/01/20 09:07:44  mohor

// CRC generation iand verification in bench changed.

//

// Revision 1.30  2004/01/20 08:03:35  mohor

// IDCODE test improved.

//

// Revision 1.29  2004/01/19 13:13:18  mohor

// Define tap_defines.v added to test bench.

//

// Revision 1.28  2004/01/19 12:38:10  mohor

// Waiting for "ready" improved.

//

// Revision 1.27  2004/01/17 18:01:31  mohor

// New version.

//

// Revision 1.26  2004/01/17 17:01:25  mohor

// Almost finished.

//

// Revision 1.25  2004/01/16 14:51:24  mohor

// cpu registers added.

//

// Revision 1.24  2004/01/15 10:47:13  mohor

// Working.

//

// Revision 1.23  2004/01/14 22:59:01  mohor

// Temp version.

//

// Revision 1.22  2004/01/13 11:28:30  mohor

// tmp version.

//

// Revision 1.21  2004/01/10 07:50:41  mohor

// temp version.

//

// Revision 1.20  2004/01/09 12:49:23  mohor

// tmp version.

//

// Revision 1.19  2004/01/08 17:53:12  mohor

// tmp version.

//

// Revision 1.18  2004/01/07 11:59:48  mohor

// temp4 version.

//

// Revision 1.17  2004/01/06 17:14:59  mohor

// temp3 version.

//

// Revision 1.16  2004/01/05 12:16:50  mohor

// tmp2 version.

//

// Revision 1.15  2003/12/23 14:26:01  mohor

// New version of the debug interface. Not finished, yet.

//

// Revision 1.14  2003/10/23 16:16:30  mohor

// CRC logic changed.

//

// Revision 1.13  2003/08/28 13:54:33  simons

// Three more chains added for cpu debug access.

//

// Revision 1.12  2002/05/07 14:44:52  mohor

// mon_cntl_o signals that controls monitor mux added.

//

// Revision 1.11  2002/03/12 14:32:26  mohor

// Few outputs for boundary scan chain added.

//

// Revision 1.10  2002/03/08 15:27:08  mohor

// Structure changed. Hooks for jtag chain added.

//

// Revision 1.9  2001/10/19 11:39:20  mohor

// dbg_timescale.v changed to timescale.v This is done for the simulation of

// few different cores in a single project.

//

// Revision 1.8  2001/10/17 10:39:17  mohor

// bs_chain_o added.

//

// Revision 1.7  2001/10/16 10:10:18  mohor

// Signal names changed to lowercase.

//

// Revision 1.6  2001/10/15 09:52:50  mohor

// Wishbone interface added, few fixes for better performance,

// hooks for boundary scan testing added.

//

// Revision 1.5  2001/09/24 14:06:12  mohor

// Changes connected to the OpenRISC access (SPR read, SPR write).

//

// Revision 1.4  2001/09/20 10:10:29  mohor

// Working version. Few bugs fixed, comments added.

//

// Revision 1.3  2001/09/19 11:54:03  mohor

// Minor changes for simulation.

//

// Revision 1.2  2001/09/18 14:12:43  mohor

// Trace fixed. Some registers changed, trace simplified.

//

// Revision 1.1.1.1  2001/09/13 13:49:19  mohor

// Initial official release.

//

// Revision 1.3  2001/06/01 22:23:40  mohor

// This is a backup. It is not a fully working version. Not for use, yet.

//

// Revision 1.2  2001/05/18 13:10:05  mohor

// Headers changed. All additional information is now avaliable in the README.txt file.

//

// Revision 1.1.1.1  2001/05/18 06:35:15  mohor

// Initial release

//

//

`include "timescale.v"

`include "tap_defines.v"

`include "dbg_defines.v"

`include "dbg_wb_defines.v"

`include "dbg_cpu_defines.v"

// Test bench

module dbg_tb;

parameter TCLK = 50;   // Clock half period (Clok period = 100 ns => 10 MHz)

reg   tms_pad_i;

reg   tck_pad_i;

reg   trst_pad_i;

reg   tdi_pad_i;

wire  tdo_pad_o;

wire  tdo_padoe_o;

wire  shift_dr_o;

wire  pause_dr_o;

wire  update_dr_o;

wire  capture_dr_o;

wire  extest_select_o;

wire  sample_preload_select_o;

wire  mbist_select_o;

wire  debug_select_o;

// WISHBONE common signals

reg   wb_rst_i;

reg   wb_clk_i;

// WISHBONE master interface

wire [31:0] wb_adr_o;

wire [31:0] wb_dat_o;

wire [31:0] wb_dat_i;

wire        wb_cyc_o;

wire        wb_stb_o;

wire  [3:0] wb_sel_o;

wire        wb_we_o;

wire        wb_ack_i;

wire        wb_cab_o;

wire        wb_err_i;

wire  [2:0] wb_cti_o;

wire  [1:0] wb_bte_o;

// CPU signals

wire        cpu_clk_i;

wire [31:0] cpu_addr_o;

wire [31:0] cpu_data_i;

wire [31:0] cpu_data_o;

wire        cpu_bp_i;

wire        cpu_stall_o;

wire        cpu_stall_all_o;

wire        cpu_stb_o;

wire  [`CPU_NUM -1:0]  cpu_sel_o;

wire        cpu_we_o;

wire        cpu_ack_i;

wire        cpu_rst_o;

// Text used for easier debugging

reg [199:0] test_text;

reg   [2:0] last_wb_cmd;

reg [199:0] last_wb_cmd_text;

reg  [31:0] wb_data [0:4095];   // Data that is written to (read from) wishbone is stored here. 

wire  tdo_o;

wire  debug_tdi_i;

wire  bs_chain_tdi_i;

wire  mbist_tdi_i;

reg   test_enabled;

reg [31:0] result;

reg [31:0] in_data_le, in_data_be;

reg [31:0] id;

wire crc_match_in;

reg [31:0] crc_in;

reg [31:0] crc_out;

wire tdo;

assign tdo = tdo_padoe_o? tdo_pad_o : 1'hz;

// Connecting TAP module

tap_top i_tap_top (

                    .tms_pad_i        (tms_pad_i),

                    .tck_pad_i        (tck_pad_i),

                    .trst_pad_i       (!trst_pad_i),

                    .tdi_pad_i        (tdi_pad_i),

                    .tdo_pad_o        (tdo_pad_o),

                    .tdo_padoe_o      (tdo_padoe_o),

                    // TAP states

                    .shift_dr_o       (shift_dr_o),

                    .pause_dr_o       (pause_dr_o),

                    .update_dr_o      (update_dr_o),

                    .capture_dr_o     (capture_dr_o),

                    // Select signals for boundary scan or mbist

                    .extest_select_o  (extest_select_o),

                    .sample_preload_select_o(sample_preload_select_o),

                    .mbist_select_o   (mbist_select_o),

                    .debug_select_o   (debug_select_o),

                    // TDO signal that is connected to TDI of sub-modules.

                    .tdo_o            (tdo_o),

                    // TDI signals from sub-modules

                    .debug_tdi_i      (debug_tdi_i),        // from debug module

                    .bs_chain_tdi_i   (bs_chain_tdi_i),  // from Boundary Scan Chain

                    .mbist_tdi_i      (mbist_tdi_i)         // from Mbist Chain

               );

// Connecting debug top module

dbg_top i_dbg_top  (

                    .tck_i            (tck_pad_i),

                    .tdi_i            (tdo_o),

                    .tdo_o            (debug_tdi_i),

                    // TAP states

                    .shift_dr_i       (shift_dr_o),

                    .pause_dr_i       (pause_dr_o),

                    .update_dr_i      (update_dr_o),

                    // Instructions

                    .debug_select_i   (debug_select_o),

                    // WISHBONE common signals

                    .wb_rst_i         (wb_rst_i),

                    .wb_clk_i         (wb_clk_i),

                    // WISHBONE master interface

                    .wb_adr_o         (wb_adr_o),

                    .wb_dat_o         (wb_dat_o),

                    .wb_dat_i         (wb_dat_i),

                    .wb_cyc_o         (wb_cyc_o),

                    .wb_stb_o         (wb_stb_o),

                    .wb_sel_o         (wb_sel_o),

                    .wb_we_o          (wb_we_o),

                    .wb_ack_i         (wb_ack_i),

                    .wb_cab_o         (wb_cab_o),

                    .wb_err_i         (wb_err_i),

                    .wb_cti_o         (wb_cti_o),

                    .wb_bte_o         (wb_bte_o),

                    // CPU signals

                    .cpu_clk_i        (cpu_clk_i),

                    .cpu_addr_o       (cpu_addr_o),

                    .cpu_data_i       (cpu_data_i),

                    .cpu_data_o       (cpu_data_o),

                    .cpu_bp_i         (cpu_bp_i),

                    .cpu_stall_o      (cpu_stall_o),

                    .cpu_stall_all_o  (cpu_stall_all_o),

                    .cpu_stb_o        (cpu_stb_o),

                    .cpu_sel_o        (cpu_sel_o),

                    .cpu_we_o         (cpu_we_o),

                    .cpu_ack_i        (cpu_ack_i),

                    .cpu_rst_o        (cpu_rst_o)

                   );

wb_slave_behavioral wb_slave

                   (

                    .CLK_I            (wb_clk_i),

                    .RST_I            (wb_rst_i),

                    .ACK_O            (wb_ack_i),

                    .ADR_I            (wb_adr_o),

                    .CYC_I            (wb_cyc_o),

                    .DAT_O            (wb_dat_i),

                    .DAT_I            (wb_dat_o),

                    .ERR_O            (wb_err_i),

                    .RTY_O            (),      // NOT USED for now!

                    .SEL_I            (wb_sel_o),

                    .STB_I            (wb_stb_o),

                    .WE_I             (wb_we_o),

                    .CAB_I            (1'b0)

                   );

cpu_behavioral i_cpu_behavioral

                   (

                    // CPU signals

                    .cpu_rst_i        (wb_rst_i),

                    .cpu_clk_o        (cpu_clk_i),

                    .cpu_addr_i       (cpu_addr_o),

                    .cpu_data_o       (cpu_data_i),

                    .cpu_data_i       (cpu_data_o),

                    .cpu_bp_o         (cpu_bp_i),

                    .cpu_stall_i      (cpu_stall_o),

                    .cpu_stall_all_i  (cpu_stall_all_o),

                    .cpu_stb_i        (cpu_stb_o),

                    .cpu_sel_i        (cpu_sel_o),

                    .cpu_we_i         (cpu_we_o),

                    .cpu_ack_o        (cpu_ack_i),

                    .cpu_rst_o        (cpu_rst_o)

                   );

// Initial values

initial

begin

  trst_pad_i = 1'b1;

  tms_pad_i = 1'hz;

  tck_pad_i = 1'hz;

  tdi_pad_i = 1'hz;

  #100;

  trst_pad_i = 1'b0;

  #100;

  trst_pad_i = 1'b1;

end

initial

begin

  test_enabled = 1'b0;

  wb_rst_i = 1'b0;

  #1000;

  wb_rst_i = 1'b1;

  #1000;

  wb_rst_i = 1'b0;

  // Initial values for wishbone slave model

  wb_slave.cycle_response(`ACK_RESPONSE, 9'h55, 8'h2);   // (`ACK_RESPONSE, wbs_waits, wbs_retries);

  #1 test_enabled<=#1 1'b1;

end

initial

begin

  wb_clk_i = 1'b0;

  forever #5 wb_clk_i = ~wb_clk_i;

end

always @ (posedge test_enabled)

begin

  $display("//////////////////////////////////////////////////////////////////");

  $display("//                                                              //");

  $display("//  (%0t) dbg_tb starting                                     //", $time);

  $display("//                                                              //");

  $display("//////////////////////////////////////////////////////////////////");

  initialize_memory(32'h12340000, 32'h00100000);  // Initialize 0x100000 bytes starting from address 0x12340000

  reset_tap;

  #500;

  goto_run_test_idle;

  // Test stall signal

  stall_test;

  // Testing read and write to internal registers

  #10000;

  set_instruction(`IDCODE);

  read_id_code(id);

  $display("\tRead ID     = 0x%0x", id);

  $display("\tExpected ID = 0x%0x", `IDCODE_VALUE);

  set_instruction(`DEBUG);

  #10000;

  chain_select(`WISHBONE_DEBUG_CHAIN, 1'b0);   // {chain, gen_crc_err}

//  #10000;

//  xxx(4'b1001, 32'he579b242);

  #10000;

//  debug_wishbone(`WB_READ8, 1'b0, 32'h12345678, 16'h4, 1'b0, "abc 1"); // {command, ready, addr, length, gen_crc_err, text}

//  debug_wishbone(`WB_READ8, 1'b0, 32'h12345679, 16'h4, 1'b0, "abc 2"); // {command, ready, addr, length, gen_crc_err, text}

//  debug_wishbone(`WB_READ8, 1'b0, 32'h1234567a, 16'h4, 1'b0, "abc 3"); // {command, ready, addr, length, gen_crc_err, text}

//

//  debug_wishbone(`WB_READ16, 1'b0, 32'h12345678, 16'h4, 1'b0, "abc 4"); // {command, ready, addr, length, gen_crc_err, text}

//  debug_wishbone(`WB_READ16, 1'b0, 32'h1234567a, 16'h4, 1'b0, "abc 5"); // {command, ready, addr, length, gen_crc_err, text}

//

  debug_wishbone(`WB_READ32, 1'b0, 32'h12345678, 16'h4, 1'b0, "read32 1"); // {command, ready, addr, length, gen_crc_err, text}

//

//  debug_wishbone(`WB_READ16, 1'b0, 32'h12345679, 16'h4, 1'b0, "abc 6"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

//  xxx(4'b1001, 32'he579b242);

  debug_wishbone(`WB_READ32, 1'b1, 32'h12345678, 16'h4, 1'b0, "read32 2"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  wb_slave.cycle_response(`ACK_RESPONSE, 9'h55, 8'h2);   // (`ACK_RESPONSE, wbs_waits, wbs_retries);

  debug_wishbone(`WB_READ32, 1'b1, 32'h12346668, 16'h4, 1'b0, "read32 3"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  wb_slave.cycle_response(`ERR_RESPONSE, 9'h03, 8'h2);   // (`ERR_RESPONSE, wbs_waits, wbs_retries);

  debug_wishbone(`WB_READ32, 1'b1, 32'h12346668, 16'h4, 1'b0, "read32 4"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_STATUS, 1'b0, 32'h0, 16'h0, 1'b0, "status 1"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_STATUS, 1'b0, 32'h0, 16'h0, 1'b0, "status 2"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  wb_slave.cycle_response(`ACK_RESPONSE, 9'h012, 8'h2);   // (`ACK_RESPONSE, wbs_waits, wbs_retries);

  debug_wishbone(`WB_READ32, 1'b1, 32'h12347778, 16'hc, 1'b0, "read32 5"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_WRITE32, 1'b0, 32'h12346668, 16'h8, 1'b0, "wr32 len8"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_WRITE16, 1'b0, 32'h12344446, 16'h8, 1'b0, "wr16 len8"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_WRITE8, 1'b0, 32'h1234010e, 16'h8, 1'b0, "wr8 len8"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_GO, 1'b0, 32'h0, 16'h0, 1'b0, "go 1"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_READ32, 1'b1, 32'h12340100, 16'hc, 1'b0, "read32 6"); // {command, ready, addr, length, gen_crc_err, text}

//  debug_wishbone(`WB_READ32, 1'b1, 32'h12340100, 16'hfffc, 1'b0, "read32 6"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

//  debug_wishbone(`WB_READ16, 1'b1, 32'h12340102, 16'he, 1'b0, "read16 7"); // {command, ready, addr, length, gen_crc_err, text}

//  debug_wishbone(`WB_READ16, 1'b1, 32'h12340102, 16'hfffe, 1'b0, "read16 7"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

//  debug_wishbone(`WB_READ8, 1'b1, 32'h12348804, 16'h6, 1'b0, "read8 8"); // {command, ready, addr, length, gen_crc_err, text}  

//  debug_wishbone(`WB_READ8, 1'b1, 32'h12348804, 16'hfffc, 1'b0, "read8 8"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  debug_wishbone(`WB_GO, 1'b0, 32'h0, 16'h0, 1'b0, "go 2"); // {command, ready, addr, length, gen_crc_err, text}

  #10000;

  chain_select(`CPU_DEBUG_CHAIN, 1'b0);   // {chain, gen_crc_err}

  // Select cpu0

  #10000;

  debug_cpu(`CPU_WRITE_REG, `CPU_SEL_ADR, 32'h0, 1'b0, result, "select cpu 0"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'h1, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // Read register

  #10000;

  debug_cpu(`CPU_READ_REG, `CPU_SEL_ADR, 32'h0, 1'b0, result, "cpu_read_reg"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'hff, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // Stall cpu0

  #10000;

  debug_cpu(`CPU_WRITE_REG, `CPU_OP_ADR, 32'h0, 1'b0, result, "stall cpu0"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'h1, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // write to cpu 32-bit

  #10000;

  debug_cpu(`CPU_WRITE32, 32'h32323232, 32'h0, 1'b0, result, "cpu_write_32"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'hdeadbeef, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // read from cpu 32-bit

  #10000;

  debug_cpu(`CPU_READ32, 32'h32323232, 32'h0, 1'b0, result, "cpu_read_32"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'hdeadbeef, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // write to cpu 8-bit

  #10000;

  debug_cpu(`CPU_WRITE8, 32'h08080808, 32'h0, 1'b0, result, "cpu_write_8"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'hdeadbeef, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  // read from cpu 8-bit

  #10000;

  debug_cpu(`CPU_READ8, 32'h08080808, 32'h0, 1'b0, result, "cpu_read_8"); // {command, addr, data, gen_crc_err, result, text}

  #10000;

  debug_cpu(`CPU_GO, 32'h0, 32'hdeadbeef, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

  #5000 gen_clk(1);            // One extra TCLK for debugging purposes

  $display("\n\nSimulation end.");

  #1000 $stop;

end

task stall_test;

  integer i;

  begin

    $display("\n\n(%0t) stall_test started", $time);

    // Set bp_i active for 1 clock cycle and check is stall is set or not

    check_stall(0); // Should not be set at the beginning

    @ (posedge wb_clk_i);

      #1 dbg_tb.i_cpu_behavioral.cpu_bp_o = 1'b1;

      check_stall(1); // set?

    @ (posedge wb_clk_i);

      #1 dbg_tb.i_cpu_behavioral.cpu_bp_o = 1'b0;

      check_stall(1); // set?

    gen_clk(1);

    check_stall(1); // set?

    // Unstall with register

    set_instruction(`DEBUG);

    chain_select(`CPU_DEBUG_CHAIN, 1'b0);   // {chain, gen_crc_err}

    check_stall(1); // set?

    debug_cpu(`CPU_WRITE_REG, `CPU_OP_ADR, 32'h0, 1'b0, result, "clr unstall"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(1); // set?

    debug_cpu(`CPU_GO, 32'h0, 32'h0, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(0); // reset?

    // Set stall with register

    debug_cpu(`CPU_WRITE_REG, `CPU_OP_ADR, 32'h0, 1'b0, result, "clr stall"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(0); // reset?

    debug_cpu(`CPU_GO, 32'h0, 32'h1, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(1); // set?

    // Unstall with register

    debug_cpu(`CPU_WRITE_REG, `CPU_OP_ADR, 32'h0, 1'b0, result, "clr unstall"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(1); // set?

    debug_cpu(`CPU_GO, 32'h0, 32'h0, 1'b0, result, "go cpu"); // {command, addr, data, gen_crc_err, result, text}

    check_stall(0); // reset?

    $display("\n\n(%0t) stall_test passed\n\n", $time);

  end

endtask   // stall_test

task check_stall;

  input should_be_set;

  begin

    if (should_be_set && (!cpu_stall_o))

      begin

        $display ("\t\t(%0t) ERROR: cpu_stall_o is not set but should be.", $time);

        $stop;

      end

    if ((!should_be_set) && cpu_stall_o)

      begin

        $display ("\t\t(%0t) ERROR: cpu_stall_o set but shouldn't be.", $time);

        $stop;

      end

  end

endtask   // check_stall

task initialize_memory;

  input [31:0] start_addr;

  input [31:0] length;

  integer i;

  reg [31:0] addr;

  begin