diff --git a/cpu.js b/cpu.js
index c420178..6f6c7db 100644
--- a/cpu.js
+++ b/cpu.js
@@ -84,7 +84,7 @@ function CPU_65816() {
                       0x65 : ADC_direct_page, 0x72 : ADC_direct_page_indirect,
                       0x7d : ADC_absolute_indexed_x, 
                       0x79 : ADC_absolute_indexed_y,
-                      0x75 : ADC_direct_page_indexed_x };
+                      0x75 : ADC_direct_page_indexed_x, 0xe9 : SBC_const };
 
   /**
    * Take a raw hex string representing the program and execute it.
@@ -163,6 +163,63 @@ var MMU = {
   }
 };
 
+var SBC_const = {
+  bytes_required:function(cpu) {
+    if(cpu.p.m) {
+      return 2;
+    } else {
+      return 3;
+    }  
+  },
+  execute:function(cpu, bytes) {
+    var old_a = cpu.r.a;
+    if(cpu.p.m) {
+      cpu.r.a -= bytes[0] - cpu.p.c;
+      if(cpu.r.a < 0) {
+        cpu.p.c = 1; 
+        cpu.r.a = 0x100 + cpu.r.a;
+      } else {
+        cpu.p.c = 0;
+      }
+      cpu.p.n = cpu.r.a >> 7;  
+
+      // Check for signed overflow.
+      // If they started with the same sign and then the resulting sign is
+      // different then we have a signed overflow.
+      if((!((old_a ^ bytes[0]) & 0x80)) && ((cpu.r.a ^ old_a) & 0x80)) {
+        cpu.p.v = 1;
+      } else {
+        cpu.p.v = 0;
+      }
+    } else {
+      var argument = (bytes[1]<<8)|bytes[0]; 
+      cpu.r.a -= argument - cpu.p.c;
+      if(cpu.r.a < 0) {
+        cpu.p.c = 1; 
+        cpu.r.a = 0x10000 + cpu.r.a;
+      } else {
+        cpu.p.c = 0;
+      }
+      cpu.p.n = cpu.r.a >> 15;  
+
+      // Check for signed overflow.
+      // If they started with the same sign and then the resulting sign is
+      // different then we have a signed overflow.
+      if((!((old_a ^ argument) & 0x8000)) && ((cpu.r.a ^ old_a) & 0x8000)) {
+        cpu.p.v = 1;
+      } else {
+        cpu.p.v = 0;
+      }     
+    } 
+
+    if(cpu.r.a===0) {
+      cpu.p.z = 1;
+    } else {
+      cpu.p.z = 0;
+    }
+  }
+};
+
 var ADC_const = {
   bytes_required:function(cpu) {
     if(cpu.p.m) {
@@ -172,8 +229,8 @@ var ADC_const = {
     }
   },
   execute:function(cpu, bytes) {
+    var old_a = cpu.r.a; 
     if(cpu.p.m) {
-      var old_a = cpu.r.a; 
       cpu.r.a += bytes[0] + cpu.p.c;
       if(cpu.r.a & 0x100) {
         cpu.p.c = 1;
@@ -192,7 +249,6 @@ var ADC_const = {
         cpu.p.v = 0;
       }
     } else {
-      var old_a = cpu.r.a;
       var argument = (bytes[1]<<8)|bytes[0];
       cpu.r.a += argument + cpu.p.c;
       if(cpu.r.a & 0x10000) {
@@ -272,7 +328,7 @@ var ADC_direct_page_indirect = {
   } 
 };
 
-ADC_absolute_indexed_x = {
+var ADC_absolute_indexed_x = {
   bytes_required:function() {
     return 3;
   },
@@ -281,7 +337,7 @@ ADC_absolute_indexed_x = {
   }
 };
 
-ADC_absolute_indexed_y = {
+var ADC_absolute_indexed_y = {
   bytes_required:function() {
     return 3;
   },
@@ -290,7 +346,7 @@ ADC_absolute_indexed_y = {
   }
 };
 
-ADC_direct_page_indexed_x = {
+var ADC_direct_page_indexed_x = {
   bytes_required:function() {
     return 2;
   },
diff --git a/test/tests.js b/test/tests.js
index c6e7ab6..048330b 100644
--- a/test/tests.js
+++ b/test/tests.js
@@ -19,6 +19,72 @@ function run_tests() {
   test_sep();
   test_branching();
   test_adc();
+  test_sbc();
+}
+
+function test_sbc() {
+  module("SBC");
+  test("Test normal subtraction of two 8-bit numbers that don't cause a "+
+       "borrow.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fbe23018a901e901");
+    equals(cpu.r.a, 0, "0x01 - 0x01 should result in zero when using "+
+                       "SBC");
+    equals(cpu.p.z, 1, "0x01 - 0x01 should set the zero(z) bit when "+
+                       "using SBC");
+    equals(cpu.p.n, 0, "0x01 - 0x01 should not set the negative(n) bit "+
+                       "when using SBC");
+    equals(cpu.p.v, 0, "0x01 - 0x01 should not set the overflow(v) bit "+
+                       "when using SBC");
+    equals(cpu.p.c, 0, "0x01 - 0x01 should not set the carry(c) bit when "+
+                       "using SBC");
+  });
+
+  test("Test normal subtraction of two 16-bit numbers that don't cause a "+
+       "borrow.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fbc23018a90100e90100");
+    equals(cpu.r.a, 0, "0x0001 - 0x0001 should result in zero when using "+
+                       "SBC");
+    equals(cpu.p.z, 1, "0x0001 - 0x0001 should set the zero(z) bit when "+
+                       "using SBC");
+    equals(cpu.p.n, 0, "0x0001 - 0x0001 should not set the negative(n) bit "+
+                       "when using SBC");
+    equals(cpu.p.v, 0, "0x0001 - 0x0001 should not set the overflow(v) bit "+
+                       "when using SBC");
+    equals(cpu.p.c, 0, "0x0001 - 0x0001 should not set the carry(c) bit "+
+                       "when using SBC");
+  });
+  test("Test subtraction that triggers a borrow with 8-bit numbers", 
+       function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fbe23018a9d0e9ef");
+    equals(cpu.r.a, 0xe1, "0xd0 - 0xef should set the accumulator to 0xe1 "+
+                          "when using SBC");
+    equals(cpu.p.n, 1,    "0xd0 - 0xef should set the negative(n) bit when "+ 
+                          "using SBC");
+    equals(cpu.p.v, 0,    "0xd0 - 0xef should not set the overflow(v) bit "+
+                          "when using SBC");
+    equals(cpu.p.z, 0,    "0xd0 - 0xef should not set the zero(z) bit when "+
+                          "using SBC");
+    equals(cpu.p.c, 1,    "0xd0 - 0xef should set the carry(c) bit when "+
+                          "using SBC");  
+  });
+  test("Test subtraction that triggers a borrow with 16-bit numbers", 
+       function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fbc23018a900d0e900ef");
+    equals(cpu.r.a, 0xe100, "0xd000 - 0xef00 should set the accumulator to "+
+                            "0xe0ff when using SBC");
+    equals(cpu.p.n, 1, "0xd000 - 0xef00 should set the negative(n) bit when "+ 
+                       "using SBC");
+    equals(cpu.p.v, 0, "0xd000 - 0xef00 should not set the overflow(v) bit "+
+                       "when using SBC");
+    equals(cpu.p.z, 0, "0xd000 - 0xef00 should not set the zero(z) bit when "+
+                       "using SBC");
+    equals(cpu.p.c, 1, "0xd000 - 0xef00 should set the carry(c) bit when "+
+                       "using SBC");  
+  });
 }
 
 function test_adc() {