examples cleanup

.gitignore

@@ -53,3 +53,4 @@ examples/asm/example/example
 examples/C/sum/sum
 examples/C/fir/fir
 
+

examples/verilog/xz/distributedmux.sv

@@ -0,0 +1,58 @@
+// xz.sv
+// David_Harris@hmc.edu 30 January 2022
+// Demonstrate impact of x and z.
+
+// load with vsim xz.sv
+
+module testbench();
+  logic [3:0] d0, d1, d2;
+  logic       s0, s1, s2;
+  tri   [3:0] y;
+
+  distributedmux dut(.d0, .d1, .d2, .s0, .s1, .s2, .y);
+
+  initial begin
+      d0 = 4'b0000; d1 = 4'b0101; // d2 unknown (xxxx)
+      s0 = 0; s1 = 0; s2 = 0; 
+      #10;  // y should be floating
+      s0 = 1;
+      #10; //y should be driven to 0000
+      s0 = 0; s1 = 1; 
+      #10; // y should be driven to 0101
+      s0 = 1;
+      #10; // y should be driven to 0x0x because of contention on bits 0 and 2
+      s0 = 0; s1 = 0; s2 = 1;
+      #10; // y should be driven to unknown because d2 is unknown
+  end
+endmodule
+
+module tristate #(parameter WIDTH=32) (
+  input  logic [WIDTH-1:0] a,
+  input  logic             en, 
+  output logic [WIDTH-1:0] y); 
+
+  assign y = en ? a : 'z;
+endmodule 
+
+module distributedmux(
+  input  logic [3:0] d0, d1, d2,
+  input  logic       s0, s1, s2,
+  output tri   [3:0] y); 
+
+  tristate #(4) t0(d0, s0, y);
+  tristate #(4) t1(d1, s1, y);
+  tristate #(4) t2(d2, s2, y);
+endmodule
+
+module gpio #(parameter WIDTH=16) (
+  input  logic [WIDTH-1:0] GPIOOutVal, GPIOEn,
+  output logic [WIDTH-1:0] GPIOInVal,
+  inout  tri   [WIDTH-1:0] GPIOPin); 
+
+  assign GPIOInVal = GPIOPin;
+  tristate #(1) ts[WIDTH-1:0](GPIOOutVal, GPIOEn, GPIOPin);
+endmodule
+
+module silly(output logic [128:0] y);
+  assign y = 'bz;
+endmodule

examples/verilog/xz/xz.sv

@@ -0,0 +1,19 @@
+// xz.sv
+// David_Harris@hmc.edu 30 January 2022
+// Demonstrate impact of x and z.
+
+// load with vsim xz.sv
+
+module xz(
+  output logic w, x, y, z);
+
+  logic p, q, r;
+
+  // let p be undriven
+  assign q = 1'bz;
+  assign r = 1'bx;
+
+  assign w = q & 1'b1;
+  assign x = q | 1'b1;
+endmodule
+