diff --git a/simulation/test_alu.cpp b/simulation/test_alu.cpp
index ed77946..91b6f0f 100644
--- a/simulation/test_alu.cpp
+++ b/simulation/test_alu.cpp
@@ -2,17 +2,112 @@
 #include "verilated.h"
 #include <iostream>
 
+const std::string ANSI_RESET = "\033[0m";
+const std::string ANSI_GREEN = "\033[38;5;2m";
+const std::string ANSI_RED = "\033[38;5;1m";
+const std::string ANSI_YELLOW = "\033[38;5;11m";
+
+class Tester {
+    private:
+        int succeeded, failed;
+        std::string name;
+    public:
+        Tester(std::string test_name) : name(test_name), succeeded(0), failed(0) {
+            printf("[ %s ] - %sstarting%s\n", name.c_str(), ANSI_YELLOW.c_str(), ANSI_RESET.c_str());
+        }
+
+        template<typename T, typename U>
+        void assert_eq(std::string test_name, T got, U expected) {
+            printf("[ %s ] - %30s - ", name.c_str(), test_name.c_str());
+            if (got == expected) {
+                printf("%ssuccess%s\n", ANSI_GREEN.c_str(), ANSI_RESET.c_str());
+                succeeded++;
+            } else {
+                printf("%sfailed%s - got %08x, expected %08x\n", ANSI_RED.c_str(), ANSI_RESET.c_str(), got, expected);
+                failed++;
+            }
+        }
+
+        ~Tester() { finish(); }
+
+        void finish() {
+            if (failed == 0) {
+                printf("[ %s ] - %sall succeeded%s, out of %d total\n", name.c_str(), ANSI_GREEN.c_str(), ANSI_RESET.c_str(), succeeded + failed);
+            } else {
+                printf("[ %s ] - %s%d failed, %d succeeded%s, out of %d total\n", name.c_str(), ANSI_RED.c_str(), failed, succeeded, ANSI_RESET.c_str(), succeeded + failed);
+            }
+        }
+};
+
 int main(int argc, char **argv) {
     VerilatedContext *vctx = new VerilatedContext;
 
-    Valu *valu = new Valu(vctx);
-    valu->A = 0x2137;
-    valu->B = 0x1234;
 
-    valu->op = 0b000;
-    valu->eval();
-    printf("A = %04x + B = %04x = O = %04x\n", valu->A, valu->B, valu->O);
-    valu->op = 0b001;
-    valu->eval();
-    printf("A = %04x - B = %04x = O = %04x\n", valu->A, valu->B, valu->O);
+    Valu *valu = new Valu(vctx);
+
+    {
+        Tester add_t("add");
+
+        valu->op = 0b000;
+        valu->A = 0x2137;
+        valu->B = 0x1234;
+        valu->eval();
+        add_t.assert_eq("0x2137 + 0x1234 == 0x336b", valu->O, 0x336b);
+        add_t.assert_eq("0x2137 + 0x1234 no overflow", valu->Fflow, 0x0);
+        add_t.assert_eq("0x2137 + 0x1234 no Fzero", valu->Fzero, 0x0);
+
+        valu->op = 0b000;
+        valu->A = 0x9;
+        valu->B = 0x10;
+        valu->eval();
+        add_t.assert_eq("0x9 + 0x10 == 0x19", valu->O, 0x19);
+        add_t.assert_eq("0x9 + 0x10 no overflow", valu->Fflow, 0x0);
+        add_t.assert_eq("0x9 + 0x10 no Fzero", valu->Fzero, 0x0);
+
+        valu->op = 0b000;
+        valu->A = 0xffffffff;
+        valu->B = 0x1;
+        valu->eval();
+        add_t.assert_eq("0xffffffff + 0x1 == 0x0", valu->O, 0x0);
+        add_t.assert_eq("0xffffffff + 0x1 has overflow", valu->Fflow, 0x1);
+        add_t.assert_eq("0xffffffff + 0x1 has Fzero", valu->Fzero, 0x1);
+    }
+
+    {
+        Tester add_t("sub");
+
+        valu->op = 0b001;
+        valu->A = 0x2137;
+        valu->B = 0x0420;
+        valu->eval();
+        add_t.assert_eq("0x2137 - 0x0420 == 0x1d17", valu->O, 0x1d17);
+        add_t.assert_eq("0x2137 - 0x1234 no underflow", valu->Fflow, 0x0);
+        add_t.assert_eq("0x2137 - 0x1234 no Fzero", valu->Fzero, 0x0);
+
+        valu->op = 0b001;
+        valu->A = 0x100;
+        valu->B = 0x200;
+        valu->eval();
+        add_t.assert_eq("0x100 - 0x200 == 0xffffff00", valu->O, 0xffffff00);
+        add_t.assert_eq("0x100 - 0x200 has underflow", valu->Fflow, 0x1);
+        add_t.assert_eq("0x100 - 0x200 no Fzero", valu->Fzero, 0x0);
+
+        valu->op = 0b001;
+        valu->A = 0x0;
+        valu->B = 0x1;
+        valu->eval();
+        add_t.assert_eq("0x0 - 0x1 == 0xffffffff", valu->O, 0xffffffff);
+        add_t.assert_eq("0x0 - 0x1 has underflow", valu->Fflow, 0x1);
+        add_t.assert_eq("0x0 - 0x1 no Fzero", valu->Fzero, 0x0);
+
+        valu->op = 0b001;
+        valu->A = 0x20;
+        valu->B = 0x20;
+        valu->eval();
+        add_t.assert_eq("0x20 - 0x20 == 0x0", valu->O, 0x0);
+        add_t.assert_eq("0x20 - 0x20 no underflow", valu->Fflow, 0x0);
+        add_t.assert_eq("0x20 - 0x20 has Fzero", valu->Fzero, 0x1);
+    }
+
+
 }
diff --git a/verilog-src/alu.v b/verilog-src/alu.v
index 3fde343..2a1465c 100644
--- a/verilog-src/alu.v
+++ b/verilog-src/alu.v
@@ -15,14 +15,17 @@ module alu(
   input [31:0] B,
   input [2:0] op,
   output [31:0] O,
-  output Cout,
+  output Fflow,
   output Fzero
 );
 
 wire subtraction = op == 3'b001;
 wire [31:0] adder_B = subtraction ? ~B : B;
+wire adder_cout;
 
-carry_select_adder a(A, adder_B, subtraction, O, Cout);
-assign Fzero = & O;
+carry_select_adder a(A, adder_B, subtraction, O, adder_cout);
+
+assign Fflow = subtraction ? ~adder_cout : adder_cout;
+assign Fzero = ~ (| O);
 
 endmodule