ADC() method turned into the MATHC() method which can be used for both the ADC and SBC op codes. CPU.clk() updated accordingly.

src/chip/MOS6502/cpu.js

 var Memory = require('../../common/memory.js');
 
 
-function ADC(cpu){
+function MATHC(cpu, m){ // To be used by both the ADC and SBC op codes.
+  // m == 0 - Add
+  // m == 1 - Subtract
   switch(cpu.__step){
     case 0:
       cpu.__mem.address = cpu.__PC;
       PCUp(cpu, 1);
       cpu.__opv = cpu.__mem.byte;
       if (cpu.__op == 0x69){ // Immediate
-          ALU(cpu, 0, cpu.__opv);
+          ALU(cpu, m, cpu.__opv);
           cpu.__op = -1; break;
       }
     case 1:
       switch (cpu.__op){
         case 0x65: // Zero Page
           cpu.__mem.address = cpu.__opv;
-          ALU(cpu, 0, cpu.__mem.byte);
+          ALU(cpu, m, cpu.__mem.byte);
           cpu.__op = -1; break;
         case 0x75: // Zero Page, X
           cpu.__opv = (cpu.__opv + cpu.__XR) & 0xFF; break;
         case 0x75: // Zero Page, X
         case 0x6D: // Absolute
           cpu.__mem.address = cpu.__opv;
-          ALU(cpu, 0, cpu.__mem.byte);
+          ALU(cpu, m, cpu.__mem.byte);
           cpu.__op = -1; break; 
         case 0x7D: // Absolute, X
         case 0x79: // Absolute, Y
           cpu.__opv = (cpu.__opv & 0xFF00) | (l & 0xFF);
           if (l < 255){
             cpu.__mem.address = cpu.__opv;
-            ALU(cpu, 0, cpu.__mem.byte);
+            ALU(cpu, m, cpu.__mem.byte);
             cpu.__op = -1;
           }
           break;
         case 0x79: // Absolute, Y
           let h = (cpu.__opv >> 8) + 1;
           cpu.__mem.address = (cpu.__opv & 0xFF) | (h << 8);
-          ALU(cpu, 0, cpu.__mem.byte);
+          ALU(cpu, m, cpu.__mem.byte);
           cpu.__op = -1; break;
         case 0x61: // Indirect, X
           cpu.__mem.address += 1;
           cpu.__opv = (cpu.__opv & 0xFF00) | (l & 0xFF);
           if (l <= 255){
             cpu.__mem.address = cpu.__opv;
-            ALU(cpu, 0, cpu.__mem.byte);
+            ALU(cpu, m, cpu.__mem.byte);
             cpu.__op = -1; break;
           }
       }
         cpu.__opv = (cpu.__opv & 0x00FF) | (h << 8);
       }
       cpu.__mem.address = cpu.__opv;
-      ALU(cpu, 0, cpu.__mem.byte);
+      ALU(cpu, m, cpu.__mem.byte);
       cpu.__op = -1; break;
   }
   cpu.__step += 1;
         } else {
           switch(this.__op){
             case 0x69: case 0x65: case 0x75: case 0x6D: case 0x7D: case 0x79: case 0x61: case 0x71:
-              ADC(this); break;
+              MATHC(this, 0); break;
             case 0x29: case 0x25: case 0x35: case 0x2D: case 0x3D: case 0x39: case 0x21: case 0x31:
               AND(this); break;
             case 0x0A: case 0x06: case 0x16: case 0x0E: case 0x1E:
             case 0x60:
               RTS(this); break;
             case 0xE9: case 0xE5: case 0xF5: case 0xED: case 0xFD: case 0xF9: case 0xE1: case 0xF1:
-              SBC(this); break;
+              MATHC(this, 1); break;
             case 0x85: case 0x95: case 0x8D: case 0x9D: case 0x99: case 0x81: case 0x91:
               STA(this); break;
             case 0x9A: case 0xBA: case 0x48: case 0x68: case 0x08: case 0x28: