Added dedicated StackPush() and StackPop() methods. Fixed BRK and JSR opcodes. Wrote RTS and RTI opcodes.

src/chip/MOS6502/cpu.js

     case 0:
       PCUp(cpu, 1);
     case 1:
-      cpu.__mem.address = cpu.__SP;
-      cpu.__mem.byte = cpu.__PC & 0xFF;
-      cpu.__SP = ByteWrap(cpu.__SP, -1);
-      break;
+      StackPush(cpu, cpu.__PC >> 8); break;
     case 2:
-      cpu.__mem.address += 1;
-      cpu.__mem.byte = cpu.__PC >> 8;
-      cpu.__SP = ByteWrap(cpu.__SP, -1);
-      break;
+      StackPush(cpu, cpu.__PC & 0xFF); break;
     case 3:
-      cpu.__mem.address += 1;
-      cpu.__mem.byte = cpu.__PR;
-      cpu.__SP = ByteWrap(cpu.__SP, -1);
+      StackPush(cpu, cpu.__SP); break;
     case 4:
       cpu.__mem.address = 0xFFFE;
       cpu.__PC = cpu.__mem.byte;
       // Discard stack data (or... No Op)
       break;
     case 2:
-      cpu.__mem.address = cpu.__SP;
-      cpu.__mem.byte = cpu.__PC & 0xFF;
-      cpu.__SP = ByteWrap(cpu.__SP, -1);
-      break;
+      StackPush(cpu, cpu.__PC >> 8); break;
     case 3:
-      cpu.__mem.address -= 1;
-      cpu.__mem.byte = cpu.__PC >> 8;
-      cpu.__SP = ByteWrap(cpu.__SP, -1);
-      break;
+      StackPush(cpu, cpu.__PC & 0xFF); break;
     case 4:
       cpu.__mem.address = cpu.__PC;
       cpu.__PC = cpu.__opv | (cpu.__mem.byte << 8)
 }
 
 function RTI(cpu){
-
+  switch(cpu.__cycle){
+    case 0: break; // Discard PC
+    case 1: // Discard Stack Pointer Data
+      StackPop(cpu); break;
+    case 2:
+      cpu.__PR = StackPop(cpu); break;
+    case 3:
+      cpu.__PC = StackPop(cpu); break;
+    case 4:
+      cpu.__PC |= StackPop(cpu) << 8;
+      cpu.__op = -1;
+      break;
+  }
+  cpu.__cycle += 1;
 }
 
 function RTS(cpu){
-
+  switch(cpu.__cycle){
+    case 0: break; // Discard PC
+    case 1:
+      StackPop(cpu); break;
+    case 2:
+      cpu.__PC = StackPop(cpu); break;
+    case 3:
+      cpu.__PC |= StackPop(cpu) << 8; break;
+    case 4:
+      PCUp(cpu, 1);
+      cpu.__op = -1;
+      break;
+  }
+  cpu.__cycle += 1;
 }
 
 function SBC(cpu){
   return v;
 }
 
+function StackPush(cpu, v){
+  cpu.__mem.address = 0x0100 | cpu.__SP;
+  cpu.__mem.byte = v & 0xFF;
+  cpu.__SP = ByteWrap(cpu.__SP, -1);
+}
+
+function StackPop(cpu){
+  cpu.__mem.address = 0x0100 | cpu.__SP;
+  cpu.__SP = ByteWrap(cpu.__SP, 1);
+  return cpu.__mem.byte;
+}
+
 function PCHI(cpu, b){
   cpu.__PC = (cpu.__PC & 0x00FF) | (b << 8);
 }