diff --git a/TODO b/TODO
index bdd8fc8..0aab6b3 100644
--- a/TODO
+++ b/TODO
@@ -9,7 +9,8 @@ Testing:
 - Tests needed for the numerous other operations added
 CPU:
 - More work on interrupts
-- Decimal mode support
+- Decimal mode support(figure out details as to how it handles illegal BCD
+  numbers)
 - Have each operation return the number of cycles used by the execute function.
 - Optimization
 Memory:
diff --git a/cpu.js b/cpu.js
index 322c45e..2983425 100644
--- a/cpu.js
+++ b/cpu.js
@@ -3610,22 +3610,46 @@ var ADC_const = {
   execute:function(cpu, bytes) {
     var old_a = cpu.r.a; 
     if(cpu.p.e|cpu.p.m) {
-      cpu.r.a += bytes[0] + cpu.p.c;
-      if(cpu.r.a & 0x100) {
-        cpu.p.c = 1;
+      if(cpu.p.d) {
+        // Form a decimal number out of a.
+        var ones = cpu.r.a & 0x0f;        
+        var tens = cpu.r.a >>4;
+        var dec_a = (tens*10)+ones;
+      
+        // Form a decimal number out of the argument.
+        ones = bytes[0] & 0x0f;
+        tens = bytes[0] >>4;
+        var dec_arg = (tens*10)+ones;
+        var result = dec_a + dec_arg;
+        // Check for decimal overflow.
+        if(result>99) {
+          result -= 99;
+          cpu.p.c = 1; 
+        }
+        var digits = result.toString(10).split("");
+        var i = 0;
+        cpu.r.a = 0;
+        for(i=0;i<digits.length;i++) {
+          cpu.r.a += (digits[i]-0)*Math.pow(16,digits.length-i-1);
+        }
       } else {
-        cpu.p.c = 0;
-      } 
-      cpu.r.a &= 0xff;
-      cpu.p.n = cpu.r.a >> 7;
+        cpu.r.a += bytes[0] + cpu.p.c;
+        if(cpu.r.a & 0x100) {
+          cpu.p.c = 1;
+        } else {
+          cpu.p.c = 0;
+        } 
+        cpu.r.a &= 0xff;
+        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;
+        // 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];
diff --git a/test/tests.js b/test/tests.js
index 13601ff..e451c9e 100644
--- a/test/tests.js
+++ b/test/tests.js
@@ -15,7 +15,7 @@
  */
 
 function run_tests() {
-  test_lda();
+  test_lda(); 
   test_rep();
   test_sep();
   test_branching();
@@ -566,6 +566,52 @@ function test_adc() {
                        "using ADC");   
 
   });
+  test("Test that ADC handles decimal mode with legal BCD numbers in 8-bit "+
+       "memory/accumulator mode with single digit numbers.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fb18f8a9056905");
+    equals(cpu.r.a, 0x10, "0x05 + 0x05 should result in 0x10 with decimal "+
+                          "mode on and with 8-bit memory/accumulator mode.");
+    equals(cpu.p.c, 0, "The carry flag of the p status register should be "+
+                       "clear after no decimal overflow.");
+    equals(cpu.p.d, 1, "Decimal mode should be set to 1 in the p status "+
+                       "register.");
+    equals(cpu.p.m, 1, "The m flag of the p status register should be 1 for "+
+                       "8-bit memory/accumulator mode.");
+    equals(cpu.p.e, 0, "The hidden e flag of the p status register should "+
+                       "be 0 for native mode.");
+  });
+  test("Test that ADC handles decimal mode with legal BCD numbers in 8-bit "+
+       "memory/accumulator mode with double digit numbers.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fb18f8a9156926");
+    equals(cpu.r.a, 0x41, "0x15 + 0x26 should result in 0x41 with decimal "+
+                          "mode on and with 8-bit memory/accumulator mode.");
+    equals(cpu.p.c, 0, "The carry flag of the p status register should be "+
+                       "clear after no decimal overflow.");
+    equals(cpu.p.d, 1, "Decimal mode should be set to 1 in the p status "+
+                       "register.");
+    equals(cpu.p.m, 1, "The m flag of the p status register should be 1 for "+
+                       "8-bit memory/accumulator mode.");
+    equals(cpu.p.e, 0, "The hidden e flag of the p status register should "+
+                       "be 0 for native mode.");
+  });
+  test("Test that ADC handles decimal mode with legal BCD numbers in 8-bit "+
+       "memory/accumulator mode when adding two numbers that cause an "+
+       "overflow.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("18fb18f8a9556960");
+    equals(cpu.r.a, 0x16, "0x55 + 0x60 should result in 0x16 with decimal "+
+                          "mode on and with 8-bit memory/accumulator mode.");
+    equals(cpu.p.c, 1, "The carry flag of the p status register should be "+
+                       "set after the decimal overflow.");
+    equals(cpu.p.d, 1, "Decimal mode should be set to 1 in the p status "+
+                       "register.");
+    equals(cpu.p.m, 1, "The m flag of the p status register should be 1 for "+
+                       "8-bit memory/accumulator mode.");
+    equals(cpu.p.e, 0, "The hidden e flag of the p status register should "+
+                       "be 0 for native mode.");
+  });
 }
 
 function test_branching() {