diff --git a/src/c0vm.js b/src/c0vm.js
index 02d8535..13338a9 100755
--- a/src/c0vm.js
+++ b/src/c0vm.js
@@ -1,10 +1,17 @@
 op = require("./opcodes");
 
+var INT_MIN = 0x80000000;
+var INT_MAX = 0x7FFFFFFF;
+
 var verbose = false;
 function log(message) {
     if (verbose) console.log(message);
 }
 
+function c0_assertion_failure(val) {
+    throw "c0 assertion failure: " + val;
+}
+
 var StackFrame = function(file, f) {
     log("Creating stack frame");
     this.stack = [];
@@ -12,7 +19,7 @@ var StackFrame = function(file, f) {
     this.program = f.code;
     this.variables = [];
     for (var i = 0; i < f.num_vars; i++)
-        variables.push(0);
+        this.variables.push(0);
     this.file = file;
 }
 
@@ -21,7 +28,7 @@ var ProgramState = function(parsed_file) {
     var main_function = parsed_file.function_pool[0];
 
     this.frame = new StackFrame(parsed_file, parsed_file.function_pool[0]);
-    this.call_stack = [this.frame];
+    this.call_stack = [];
     this.file = parsed_file;
 }
 
@@ -33,13 +40,23 @@ ProgramState.prototype.pop = function() {
     return this.frame.stack.pop();
 }
 
+ProgramState.prototype.goto_offset = function() {
+    var c1 = this.frame.program[this.frame.pc+1];
+    var c2 = this.frame.program[this.frame.pc+2]
+
+    var address_offset = (c1 << 8) + c2;
+    // Handle negative values
+    if ((address_offset & 0x8000) != 0)
+        address_offset = -0x8000 + (address_offset & 0x7FFF);
+
+    this.frame.pc += address_offset;
+}
+
 ProgramState.prototype.doIf = function(f) {
     var y = this.pop();
     var x = this.pop();
     if (f(x,y)) {
-        var address_offset = (this.frame.program[this.frame.pc+1] * 0x1000) + 
-            this.frame.program[this.frame.pc+2];
-        this.frame.pc += address_offset;
+        this.goto_offset();
     } else {
         this.frame.pc += 3;
     }
@@ -47,7 +64,7 @@ ProgramState.prototype.doIf = function(f) {
 
 ProgramState.prototype.step = function() {
     var opcode = this.frame.program[this.frame.pc]
-    log("Running opcode " +
+    log("0x" + this.frame.pc.toString(16) + " Running opcode " +
         op.lookup_table[opcode]);
     switch (opcode) {
         // Stack manipulation
@@ -81,8 +98,13 @@ ProgramState.prototype.step = function() {
         // Returning from a function
     case op.RETURN:
         var retVal = this.pop();
-        throw retVal;
+        if (this.call_stack.length == 0)
+            throw retVal;
 
+        this.frame = this.call_stack.pop();
+        this.push(retVal);
+
+        break;
         // Arithmetic
     case op.IADD:
         this.frame.pc++;
@@ -200,20 +222,20 @@ ProgramState.prototype.step = function() {
     case op.IF_CMPNE:
         this.doIf(function (x,y) {return y != x;});
         break;
-    case op.IF_CMPLT:
+    case op.IF_ICMPLT:
         this.doIf(function (x,y) {return y > x;});
         break;
-    case op.IF_CMPGE:
+    case op.IF_ICMPGE:
         this.doIf(function (x,y) {return y <= x;});
         break;
-    case op.IF_CMPGT:
+    case op.IF_ICMPGT:
         this.doIf(function (x,y) {return y < x;});
         break;
-    case op.IF_CMPLE:
+    case op.IF_ICMPLE:
         this.doIf(function (x,y) {return y >= x;});
         break;
     case op.GOTO:
-        this.doIf(function (x,y) {return true;});
+        this.goto_offset();
         break;
     case op.ATHROW:
         this.frame.pc++;
@@ -226,8 +248,34 @@ ProgramState.prototype.step = function() {
             c0_assertion_failure(a);
         break;
 
+        // Function call operations
+
+    case op.INVOKESTATIC:
+        var c1 = this.frame.program[this.frame.pc+1];
+        var c2 = this.frame.program[this.frame.pc+2];
+        this.frame.pc += 3;
+
+        var index = (c1 << 8) + c2;
+
+        var f = this.file.function_pool[index];
+        var newFrame = new StackFrame(this.file, f);
+        for (var i = f.num_args - 1; i >= 0; i--) {
+            newFrame.variables[i] = this.pop();
+        }
+
+        this.call_stack.push(this.frame);
+        this.frame = newFrame;
+        break;
+
     default:
-        console.log("Error: Unknown opcode: 0x" + opcode.toString(16) + "\n");
+        var opcode_name;
+        try {
+            opcode_name = op.lookup_table[opcode];
+        } catch (ignored) {
+            opcode_name = "UNKNOWN";
+        }
+        console.log("Error: Unknown opcode: 0x" + opcode.toString(16) +
+                    " (" + opcode_name + ")\n");
         throw "Error - unknown opcode";
     }
     return false;
diff --git a/src/opcodes.js b/src/opcodes.js
index b0ed6eb..98ec086 100644
--- a/src/opcodes.js
+++ b/src/opcodes.js
@@ -90,12 +90,12 @@ exports.lookup_table = {
     0x13: "ILDC",
     0x14: "ALDC",
     0x00: "NOP",
-    0x9F: "IF",
-    0xA0: "IF",
-    0xA1: "IF",
-    0xA2: "IF",
-    0xA3: "IF",
-    0xA4: "IF",
+    0x9F: "IF_CMPEQ",
+    0xA0: "IF_CMPNE",
+    0xA1: "IF_ICMPLT",
+    0xA2: "IF_ICMPGE",
+    0xA3: "IF_ICMPGT",
+    0xA4: "IF_ICMPLE",
     0xA7: "GOTO",
     0xBF: "ATHROW",
     0xCF: "ASSERT",
diff --git a/test/dsquared.c0 b/test/dsquared.c0
new file mode 100644
index 0000000..860baae
--- /dev/null
+++ b/test/dsquared.c0
@@ -0,0 +1,23 @@
+int dsquared(int x, int y)
+{
+  return x*x + y*y;
+}
+
+int main () 
+{
+  int a1 = 15;
+  int b1 = 122;
+  int a2 = 42;
+  int b2 = 0;
+  int a3 = -7;
+  int b3 = -11;
+  int a4 = -4;
+  int b4 = 3;
+  int sum = 0;
+  sum = sum + dsquared(a1,b1);
+  sum = sum + dsquared(a2,b2);
+  sum = sum + dsquared(a3,b3);
+  sum = sum + dsquared(a4,b4);
+  return sum;
+}
+
diff --git a/test/dsquared.c0.bc0 b/test/dsquared.c0.bc0
new file mode 100644
index 0000000..d95b37a
--- /dev/null
+++ b/test/dsquared.c0.bc0
@@ -0,0 +1,78 @@
+C0 C0 FF EE       # magic number
+00 09             # version 4, arch = 1 (64 bits)
+
+00 00             # int pool count
+# int pool
+
+00 00             # string pool total size
+# string pool
+
+00 02             # function count
+# function_pool
+
+#<main>
+00 00             # number of arguments = 0
+00 09             # number of local variables = 9
+00 57             # code length = 87 bytes
+10 0F    # bipush 15       # 15
+36 00    # vstore 0        # a1 = 15;
+10 7A    # bipush 122      # 122
+36 01    # vstore 1        # b1 = 122;
+10 2A    # bipush 42       # 42
+36 02    # vstore 2        # a2 = 42;
+10 00    # bipush 0        # 0
+36 03    # vstore 3        # b2 = 0;
+10 F9    # bipush -7       # -7
+36 04    # vstore 4        # a3 = -(7);
+10 F5    # bipush -11      # -11
+36 05    # vstore 5        # b3 = -(11);
+10 FC    # bipush -4       # -4
+36 06    # vstore 6        # a4 = -(4);
+10 03    # bipush 3        # 3
+36 07    # vstore 7        # b4 = 3;
+10 00    # bipush 0        # 0
+36 08    # vstore 8        # sum = 0;
+15 08    # vload 8         # sum
+15 00    # vload 0         # a1
+15 01    # vload 1         # b1
+B8 00 01 # invokestatic 1  # dsquared(a1, b1)
+60       # iadd            # (sum + dsquared(a1, b1))
+36 08    # vstore 8        # sum = (sum + dsquared(a1, b1));
+15 08    # vload 8         # sum
+15 02    # vload 2         # a2
+15 03    # vload 3         # b2
+B8 00 01 # invokestatic 1  # dsquared(a2, b2)
+60       # iadd            # (sum + dsquared(a2, b2))
+36 08    # vstore 8        # sum = (sum + dsquared(a2, b2));
+15 08    # vload 8         # sum
+15 04    # vload 4         # a3
+15 05    # vload 5         # b3
+B8 00 01 # invokestatic 1  # dsquared(a3, b3)
+60       # iadd            # (sum + dsquared(a3, b3))
+36 08    # vstore 8        # sum = (sum + dsquared(a3, b3));
+15 08    # vload 8         # sum
+15 06    # vload 6         # a4
+15 07    # vload 7         # b4
+B8 00 01 # invokestatic 1  # dsquared(a4, b4)
+60       # iadd            # (sum + dsquared(a4, b4))
+36 08    # vstore 8        # sum = (sum + dsquared(a4, b4));
+15 08    # vload 8         # sum
+B0       # return          # 
+
+
+#<dsquared>
+00 02             # number of arguments = 2
+00 02             # number of local variables = 2
+00 0C             # code length = 12 bytes
+15 00    # vload 0         # x
+15 00    # vload 0         # x
+68       # imul            # (x * x)
+15 01    # vload 1         # y
+15 01    # vload 1         # y
+68       # imul            # (y * y)
+60       # iadd            # ((x * x) + (y * y))
+B0       # return          # 
+
+00 00             # native count
+# native pool
+
diff --git a/test/dsquared.c0.bc0out b/test/dsquared.c0.bc0out
new file mode 100644
index 0000000..52d2ba8
--- /dev/null
+++ b/test/dsquared.c0.bc0out
@@ -0,0 +1 @@
+17068
diff --git a/test/dsquared.c0.c0out b/test/dsquared.c0.c0out
new file mode 100644
index 0000000..52d2ba8
--- /dev/null
+++ b/test/dsquared.c0.c0out
@@ -0,0 +1 @@
+17068
diff --git a/test/dsquared.c0.ex b/test/dsquared.c0.ex
new file mode 100755
index 0000000..60c7863
Binary files /dev/null and b/test/dsquared.c0.ex differ
diff --git a/test/isqrt.c0 b/test/isqrt.c0
new file mode 100644
index 0000000..b628688
--- /dev/null
+++ b/test/isqrt.c0
@@ -0,0 +1,10 @@
+int main () {
+  int n = 15122;
+  int i = 0; int k = 0;
+  while (k <= n) {
+    k += 2*i + 1;
+    i++;
+  }
+  return i-1;
+}
+
diff --git a/test/isqrt.c0.bc0 b/test/isqrt.c0.bc0
new file mode 100644
index 0000000..2a42549
--- /dev/null
+++ b/test/isqrt.c0.bc0
@@ -0,0 +1,51 @@
+C0 C0 FF EE       # magic number
+00 09             # version 4, arch = 1 (64 bits)
+
+00 01             # int pool count
+# int pool
+00 00 3B 12
+
+00 00             # string pool total size
+# string pool
+
+00 01             # function count
+# function_pool
+
+#<main>
+00 00             # number of arguments = 0
+00 03             # number of local variables = 3
+00 34             # code length = 52 bytes
+13 00 00 # ildc 0          # c[0] = 15122
+36 00    # vstore 0        # n = 15122;
+10 00    # bipush 0        # 0
+36 01    # vstore 1        # i = 0;
+10 00    # bipush 0        # 0
+36 02    # vstore 2        # k = 0;
+# <00:loop>
+15 02    # vload 2         # k
+15 00    # vload 0         # n
+A4 00 06 # if_icmple +6    # if (k <= n) goto <01:body>
+A7 00 1A # goto +26        # goto <02:exit>
+# <01:body>
+15 02    # vload 2         # k
+10 02    # bipush 2        # 2
+15 01    # vload 1         # i
+68       # imul            # (2 * i)
+10 01    # bipush 1        # 1
+60       # iadd            # ((2 * i) + 1)
+60       # iadd            # 
+36 02    # vstore 2        # k += ((2 * i) + 1);
+15 01    # vload 1         # i
+10 01    # bipush 1        # 1
+60       # iadd            # 
+36 01    # vstore 1        # i += 1;
+A7 FF E2 # goto -30        # goto <00:loop>
+# <02:exit>
+15 01    # vload 1         # i
+10 01    # bipush 1        # 1
+64       # isub            # (i - 1)
+B0       # return          # 
+
+00 00             # native count
+# native pool
+
diff --git a/test/isqrt.c0.bc0out b/test/isqrt.c0.bc0out
new file mode 100644
index 0000000..e69de29
diff --git a/test/isqrt.c0.c0out b/test/isqrt.c0.c0out
new file mode 100644
index 0000000..9f54fe3
--- /dev/null
+++ b/test/isqrt.c0.c0out
@@ -0,0 +1 @@
+122
diff --git a/test/isqrt.c0.ex b/test/isqrt.c0.ex
new file mode 100755
index 0000000..40a9a81
Binary files /dev/null and b/test/isqrt.c0.ex differ
diff --git a/test/mid.c0 b/test/mid.c0
new file mode 100644
index 0000000..c3efe9a
--- /dev/null
+++ b/test/mid.c0
@@ -0,0 +1,10 @@
+int main()
+{
+	int low = 0x15;
+	int high = 0x122;
+	int mid;
+
+	mid = low + (high - low)/2;
+	return mid;
+}
+
diff --git a/test/mid.c0.bc0 b/test/mid.c0.bc0
new file mode 100644
index 0000000..0d2323d
--- /dev/null
+++ b/test/mid.c0.bc0
@@ -0,0 +1,35 @@
+C0 C0 FF EE       # magic number
+00 09             # version 4, arch = 1 (64 bits)
+
+00 01             # int pool count
+# int pool
+00 00 01 22
+
+00 00             # string pool total size
+# string pool
+
+00 01             # function count
+# function_pool
+
+#<main>
+00 00             # number of arguments = 0
+00 03             # number of local variables = 3
+00 19             # code length = 25 bytes
+10 15    # bipush 21       # 21
+36 00    # vstore 0        # low = 21;
+13 00 00 # ildc 0          # c[0] = 290
+36 01    # vstore 1        # high = 290;
+15 00    # vload 0         # low
+15 01    # vload 1         # high
+15 00    # vload 0         # low
+64       # isub            # (high - low)
+10 02    # bipush 2        # 2
+6C       # idiv            # ((high - low) / 2)
+60       # iadd            # (low + ((high - low) / 2))
+36 02    # vstore 2        # mid = (low + ((high - low) / 2));
+15 02    # vload 2         # mid
+B0       # return          # 
+
+00 00             # native count
+# native pool
+
diff --git a/test/mid.c0.bc0out b/test/mid.c0.bc0out
new file mode 100644
index 0000000..bb79365
--- /dev/null
+++ b/test/mid.c0.bc0out
@@ -0,0 +1 @@
+155
diff --git a/test/mid.c0.c0out b/test/mid.c0.c0out
new file mode 100644
index 0000000..bb79365
--- /dev/null
+++ b/test/mid.c0.c0out
@@ -0,0 +1 @@
+155
diff --git a/test/mid.c0.ex b/test/mid.c0.ex
new file mode 100755
index 0000000..07c414a
Binary files /dev/null and b/test/mid.c0.ex differ
diff --git a/test/sample2.5.c0 b/test/sample2.5.c0
new file mode 100644
index 0000000..0a6bccb
--- /dev/null
+++ b/test/sample2.5.c0
@@ -0,0 +1,7 @@
+int main () {
+	int i; int sum = 0;
+	for (i = 15; i <= 122; i++)
+		sum += i;
+	assert(sum == 0);
+	return sum;
+}
diff --git a/test/sample2.5.c0.bc0 b/test/sample2.5.c0.bc0
new file mode 100644
index 0000000..6a97909
--- /dev/null
+++ b/test/sample2.5.c0.bc0
@@ -0,0 +1,55 @@
+C0 C0 FF EE       # magic number
+00 09             # version 4, arch = 1 (64 bits)
+
+00 00             # int pool count
+# int pool
+
+00 25             # string pool total size
+# string pool
+73 61 6D 70 6C 65 32 2E 35 2E 63 30 3A 35 2E 32 2D 35 2E 31 39 3A 20 61 73 73 65 72 74 20 66 61 69 6C 65 64 00  # "sample2.5.c0:5.2-5.19: assert failed"
+
+00 01             # function count
+# function_pool
+
+#<main>
+00 00             # number of arguments = 0
+00 02             # number of local variables = 2
+00 3B             # code length = 59 bytes
+10 00    # bipush 0        # 0
+36 01    # vstore 1        # sum = 0;
+10 0F    # bipush 15       # 15
+36 00    # vstore 0        # i = 15;
+# <00:loop>
+15 00    # vload 0         # i
+10 7A    # bipush 122      # 122
+A4 00 06 # if_icmple +6    # if (i <= 122) goto <01:body>
+A7 00 14 # goto +20        # goto <02:exit>
+# <01:body>
+15 01    # vload 1         # sum
+15 00    # vload 0         # i
+60       # iadd            # 
+36 01    # vstore 1        # sum += i;
+15 00    # vload 0         # i
+10 01    # bipush 1        # 1
+60       # iadd            # 
+36 00    # vstore 0        # i += 1;
+A7 FF E8 # goto -24        # goto <00:loop>
+# <02:exit>
+15 01    # vload 1         # sum
+10 00    # bipush 0        # 0
+9F 00 06 # if_cmpeq +6     # if (sum == 0) goto <03:cond_true>
+A7 00 08 # goto +8         # goto <04:cond_false>
+# <03:cond_true>
+10 01    # bipush 1        # true
+A7 00 05 # goto +5         # goto <05:cond_end>
+# <04:cond_false>
+10 00    # bipush 0        # false
+# <05:cond_end>
+14 00 00 # aldc 0          # s[0] = "sample2.5.c0:5.2-5.19: assert failed"
+CF       # assert          # assert(sum == 0) [failure message on stack]
+15 01    # vload 1         # sum
+B0       # return          # 
+
+00 00             # native count
+# native pool
+
diff --git a/test/sample2.5.c0.bc0out b/test/sample2.5.c0.bc0out
new file mode 100644
index 0000000..e69de29
diff --git a/test/sample2.5.c0.c0out b/test/sample2.5.c0.c0out
new file mode 100644
index 0000000..e69de29
diff --git a/test/sample2.5.c0.ex b/test/sample2.5.c0.ex
new file mode 100755
index 0000000..f3055ed
Binary files /dev/null and b/test/sample2.5.c0.ex differ
diff --git a/test/testif.c0 b/test/testif.c0
new file mode 100644
index 0000000..173b796
--- /dev/null
+++ b/test/testif.c0
@@ -0,0 +1,8 @@
+int main() {
+	int x = 4;
+	if (x < 3)
+		x = 23;
+	else
+		x = 32;
+	return x;
+}
diff --git a/test/testif.c0.bc0 b/test/testif.c0.bc0
new file mode 100644
index 0000000..578e4be
--- /dev/null
+++ b/test/testif.c0.bc0
@@ -0,0 +1,36 @@
+C0 C0 FF EE       # magic number
+00 09             # version 4, arch = 1 (64 bits)
+
+00 00             # int pool count
+# int pool
+
+00 00             # string pool total size
+# string pool
+
+00 01             # function count
+# function_pool
+
+#<main>
+00 00             # number of arguments = 0
+00 01             # number of local variables = 1
+00 1C             # code length = 28 bytes
+10 04    # bipush 4        # 4
+36 00    # vstore 0        # x = 4;
+15 00    # vload 0         # x
+10 03    # bipush 3        # 3
+A1 00 06 # if_icmplt +6    # if (x < 3) goto <00:then>
+A7 00 0A # goto +10        # goto <01:else>
+# <00:then>
+10 17    # bipush 23       # 23
+36 00    # vstore 0        # x = 23;
+A7 00 07 # goto +7         # goto <02:endif>
+# <01:else>
+10 20    # bipush 32       # 32
+36 00    # vstore 0        # x = 32;
+# <02:endif>
+15 00    # vload 0         # x
+B0       # return          # 
+
+00 00             # native count
+# native pool
+
diff --git a/test/testif.c0.bc0out b/test/testif.c0.bc0out
new file mode 100644
index 0000000..f5c8955
--- /dev/null
+++ b/test/testif.c0.bc0out
@@ -0,0 +1 @@
+32
diff --git a/test/testif.c0.c0out b/test/testif.c0.c0out
new file mode 100644
index 0000000..f5c8955
--- /dev/null
+++ b/test/testif.c0.c0out
@@ -0,0 +1 @@
+32
diff --git a/test/testif.c0.ex b/test/testif.c0.ex
new file mode 100755
index 0000000..10fa75d
Binary files /dev/null and b/test/testif.c0.ex differ
diff --git a/test/tests.js b/test/tests.js
index 334bf81..6229ddb 100644
--- a/test/tests.js
+++ b/test/tests.js
@@ -3,12 +3,17 @@ c0vm = require("../src/c0vm.js")
 
 var callbacks = {}
 
-// Tests that IADD, BIPUSH, and RETURN (from main) work
-exports.testIADD = function(test) {
-    var result = c0vm.execute(parser.parse("iadd.c0.bc0"), callbacks, false);
-    test.ok(result == -2, "iadd.c0.bc0 - Error in IADD, BIPUSH, or RETURN");
-    test.done();
-};
+function doTest(filename, expected_result) {
+    return function(test) {
+        var result = c0vm.execute(parser.parse(filename), callbacks, false);
+        test.ok(result == expected_result,
+                filename + " - did not get expected result " + expected_result +
+               ", instead got " + result);
+        test.done();
+    }
+}
+
+exports.testIADD = doTest("iadd.c0.bc0", -2);
 
 exports.testPrint = function(test) {
     var result = c0vm.execute(parser.parse("test.bc0"), callbacks, false);
@@ -16,8 +21,19 @@ exports.testPrint = function(test) {
     test.done();
 }
 
-exports.testISHR = function(test) {
-    var result = c0vm.execute(parser.parse("ishr.c0.bc0"), callbacks, false);
-    test.ok(result == -1, "ishr.c0.bc0 - -1 >> 1 gave " + result + ", not -1");
+exports.testISHR = doTest("ishr.c0.bc0", -1);
+
+exports.testISQRT = doTest("isqrt.c0.bc0", 122);
+
+exports.testMID = doTest("mid.c0.bc0", 155);
+
+// This one should throw an exception because an assertion fails
+exports.testSample25 = function(test) {
+    test.throws(c0vm.execute(parser.parse("sample2.5.c0.bc0"), callbacks, false));
     test.done();
 }
+
+exports.testIF = doTest("testif.c0.bc0", 32);
+
+exports.testDSQUARED = doTest("dsquared.c0.bc0", 17068);
+