Enhanced ALU with BCD mode and fixed ADC/SBC flag results.

src/MOS6502/cpu.js

 }
 
 function ALU(cpu, m, b){
-  switch(m){
-    case 0: // Addition
-      cpu.__AR = (cpu.__AR + b) + ((cpu.C === 1) ? 1 : 0);
-      //cpu.C = (this.__AR >= 256);
-      break;
-    case 1: // Subtraction
-      cpu.__AR = (cpu.__AR - b) - ((cpu.C === 0) ? 1 : 0); 
-      //cpu.C = (this.__AR >= 0);
-      cpu.__AR += (cpu.__AR < 0) ? 256 : 0;
-      break;
-  } 
-  cpu.V = (BITM.isOn(cpu.__AR, 7) === BITM.isOn(b, 7)) && (BITM.val(cpu.__AR, 7) !== BITM.val(cpu.__AR, 7));
-  cpu.C = (m === 0) ? (this.__AR >= 256) : (cpu.V === 0);
-  cpu.N = BITM.val(cpu.__AR, 7);
-  
-  cpu.__AR &= 0xFF;
-  cpu.Z = (cpu.__AR === 0);
+  let A = 0;
+  let IAR = cpu.__AR;
+  if (cpu.D === 0){ // Binary Mode
+    switch(m){
+      case 0: // Addition
+        A = (cpu.__AR + b) + ((cpu.C === 1) ? 1 : 0);
+        //cpu.C = (this.__AR >= 256);
+        break;
+      case 1: // Subtraction
+        A = (cpu.__AR - b) - ((cpu.C === 0) ? 1 : 0); 
+        //cpu.C = (this.__AR >= 0);
+        A += (A < 0) ? 256 : 0;
+        break;
+    } 
+ 
+    cpu.__AR = A & 0xFF;
+
+    // Both inputs have the same sign, but the output doesn't match the sign of the input.
+    // (Twos Compliment)
+    cpu.V = ((IAR & 0x80) === (b & 0x80)) && ((IAR & 0x80) !== (cpu.__AR & 0x80));
+    //cpu.V = (BITM.isOn(cpu.__AR, 7) === BITM.isOn(b, 7)) && (BITM.val(cpu.__AR, 7) !== BITM.val(cpu.__AR, 7));
+    cpu.C = (m === 0) ? (A >= 256) : (cpu.V === 0);
+    cpu.N = BITM.isOn(cpu.__AR, 7);
+    
+    //cpu.__AR &= 0xFF;
+    cpu.Z = (cpu.__AR === 0);
+  } else { // Decimal (BCD) Mode
+    // Borrowed from information located on...
+    // http://www.6502.org/tutorials/decimal_mode.html
+    let AL = 0;
+    switch(m){
+      case 0: // Addition 
+        AL = (cpu.__AR & 0x0F) + (b & 0x0F) + ((cpu.C === 1) ? 1 : 0);
+        AL = (AL < 0x0A) ? AL : ((AL + 0x06) & 0x0F) + 0x10;
+        A = (cpu.__AR & 0xF0) + (b & 0xF0) + AL;
+        A += (A >= 0xA0) ? 0x60 : 0;
+        cpu.__AR = A & 0xFF;
+        cpu.C = (A >= 0x100);
+        cpu.N = (BITM.isOn(A, 7)) ? 1 : 0;
+        cpu.V = (A >= -128 && A <= 127) ? 0 : 1;
+        break;
+      case 1: // Subtraction
+        AL = (cpu.__AR & 0x0F) - (b & 0x0F) + (cpu.C - 1);
+        AL = (AL >= 0) ? AL : ((AL - 0x06) & 0x0F) - 0x10;
+        A = (A & 0xF0) - (b & 0xF0) + AL;
+        A -= (A >= 0) ? 0 : 0x60;
+        cpu.__AR = A & 0xFF;
+
+        cpu.V = ((IAR & 0x80) === (b & 0x80)) && ((IAR & 0x80) !== (cpu.__AR & 0x80));
+        cpu.C = (cpu.V === 0);
+        cpu.N = BITM.isOn(cpu.__AR, 7);
+        cpu.Z = (cpu.__AR === 0);
+        break;
+    }
+  }
   return cpu.__AR;
 }