ALU() method modified to directly use and set the cpu.__AR value. ADC() Adjusted according to the new ALU() changes.

src/chip/MOS6502/cpu.js

       PCUp(cpu, 1);
       cpu.__opv = cpu.__mem.byte;
       if (cpu.__op == 0x69){ // Immediate
-          cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__opv);
+          ALU(cpu, 0, cpu.__opv);
           cpu.__op = -1; break;
       }
     case 1:
       switch (cpu.__op){
         case 0x65: // Zero Page
           cpu.__mem.address = cpu.__opv;
-          cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+          ALU(cpu, 0, 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;
-          cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+          ALU(cpu, 0, 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;
-            cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+            ALU(cpu, 0, 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);
-          cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+          ALU(cpu, 0, 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;
-            cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+            ALU(cpu, 0, cpu.__mem.byte);
             cpu.__op = -1; break;
           }
       }
         cpu.__opv = (cpu.__opv & 0x00FF) | (h << 8);
       }
       cpu.__mem.address = cpu.__opv;
-      cpu.__AR = ALU(cpu, 0, cpu.__AR, cpu.__mem.byte);
+      ALU(cpu, 0, cpu.__mem.byte);
       cpu.__op = -1; break;
   }
   cpu.__step += 1;
   return v;
 }
 
-function ALU(cpu, m, a, b){
+function ALU(cpu, m, b){
   let v = 0;
   switch(m){
     case 0: // Addition
-      v = (a + b) + ((cpu.C === 1) ? 1 : 0);
+      cpu.__AR = (cpu.__AR + b) + ((cpu.C === 1) ? 1 : 0);
       cpu.C = (v >= 256);
       break;
     case 1: // Subtraction
-      v = (a - b) - ((cpu.C === 0) ? 1 : 0);
+      cpu.__AR = (cpu.__AR - b) - ((cpu.C === 0) ? 1 : 0);
       cpu.C = (v >= 0);
       break;
   }
-  v &= 0xFF;
-  cpu.V = (BIT.isOn(a, 7) === BIT.isOn(b, 7)) && (BIT.val(v, 7) !== BIT.val(a, 7));
-  cpu.N = BIT.val(v, 7);
-  cpu.Z = (v === 0);
-  return v;
+  cpu.__AR &= 0xFF;
+  cpu.V = (BIT.isOn(cpu.__AR, 7) === BIT.isOn(b, 7)) && (BIT.val(v, 7) !== BIT.val(cpu.__AR, 7));
+  cpu.N = BIT.val(cpu.__AR, 7);
+  cpu.Z = (cpu.__AR === 0);
+  return cpu.__AR;
 }
 
 // --------------------------------------------------------------------------------------------