{"task_id": "m2014_q4j", "task_number": 39, "description": "Implement a 4-bit adder with full adders. The output sum should include the overflow bit.", "header": "module top_module (\n\tinput [3:0] x,\n\tinput [3:0] y,\n\toutput [4:0] sum\n);\n", "module_code": "module top_module (\n\tinput [3:0] x,\n\tinput [3:0] y,\n\toutput [4:0] sum\n);\n\n\tassign sum = x+y;\nendmodule\n", "testbench": "`timescale 1 ps/1 ps\n`define OK 12\n`define INCORRECT 13\nmodule reference_module (\n\tinput [3:0] x,\n\tinput [3:0] y,\n\toutput [4:0] sum\n);\n\n\tassign sum = x+y;\nendmodule\n\n\nmodule stimulus_gen (\n\tinput clk,\n\toutput logic [3:0] x,y\n);\n\n\tinitial begin\n\t\trepeat(100) @(posedge clk, negedge clk) begin\n\t\t\t{x,y} <= $random;\n\t\tend\n\t\t\n\t\t#1 $finish;\n\tend\n\t\nendmodule\n\nmodule tb();\n\n\ttypedef struct packed {\n\t\tint errors;\n\t\tint errortime;\n\t\tint errors_sum;\n\t\tint errortime_sum;\n\n\t\tint clocks;\n\t} stats;\n\t\n\tstats stats1;\n\t\n\t\n\twire[511:0] wavedrom_title;\n\twire wavedrom_enable;\n\tint wavedrom_hide_after_time;\n\t\n\treg clk=0;\n\tinitial forever\n\t\t#5 clk = ~clk;\n\n\tlogic [3:0] x;\n\tlogic [3:0] y;\n\tlogic [4:0] sum_ref;\n\tlogic [4:0] sum_dut;\n\n\tinitial begin \n\t\t$dumpfile(\"wave.vcd\");\n\t\t$dumpvars(1, stim1.clk, tb_mismatch ,x,y,sum_ref,sum_dut );\n\tend\n\n\n\twire tb_match;\t\t// Verification\n\twire tb_mismatch = ~tb_match;\n\t\n\tstimulus_gen stim1 (\n\t\t.clk,\n\t\t.* ,\n\t\t.x,\n\t\t.y );\n\treference_module good1 (\n\t\t.x,\n\t\t.y,\n\t\t.sum(sum_ref) );\n\t\t\n\ttop_module top_module1 (\n\t\t.x,\n\t\t.y,\n\t\t.sum(sum_dut) );\n\n\t\n\tbit strobe = 0;\n\ttask wait_for_end_of_timestep;\n\t\trepeat(5) begin\n\t\t\tstrobe <= !strobe;  // Try to delay until the very end of the time step.\n\t\t\t@(strobe);\n\t\tend\n\tendtask\t\n\n\t\n\tfinal begin\n\t\tif (stats1.errors_sum) $display(\"Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.\", \"sum\", stats1.errors_sum, stats1.errortime_sum);\n\t\telse $display(\"Hint: Output '%s' has no mismatches.\", \"sum\");\n\n\t\t$display(\"Hint: Total mismatched samples is %1d out of %1d samples\\n\", stats1.errors, stats1.clocks);\n\t\t$display(\"Simulation finished at %0d ps\", $time);\n\t\t$display(\"Mismatches: %1d in %1d samples\", stats1.errors, stats1.clocks);\n\tend\n\t\n\t// Verification: XORs on the right makes any X in good_vector match anything, but X in dut_vector will only match X.\n\tassign tb_match = ( { sum_ref } === ( { sum_ref } ^ { sum_dut } ^ { sum_ref } ) );\n\t// Use explicit sensitivity list here. @(*) causes NetProc::nex_input() to be called when trying to compute\n\t// the sensitivity list of the @(strobe) process, which isn't implemented.\n\talways @(posedge clk, negedge clk) begin\n\n\t\tstats1.clocks++;\n\t\tif (!tb_match) begin\n\t\t\tif (stats1.errors == 0) stats1.errortime = $time;\n\t\t\tstats1.errors++;\n\t\tend\n\t\tif (sum_ref !== ( sum_ref ^ sum_dut ^ sum_ref ))\n\t\tbegin if (stats1.errors_sum == 0) stats1.errortime_sum = $time;\n\t\t\tstats1.errors_sum = stats1.errors_sum+1'b1; end\n\n\tend\nendmodule\n"}