Fix register shift instructions

Author: coocos

src/instructions.ts

@@ -193,10 +193,14 @@ export function betweenRegisters(opcode: number, vm: VM) {
       vm.registers[0xf] = sub < 0 ? 0 : 1;
       break;
     case 0x6:
-      //Shift value of register y right by one bit and assign it to register x
-      //Store the least significant bit of register y in VF
-      vm.registers[register1] = vm.registers[register2] >> 1;
-      vm.registers[0xf] = vm.registers[register2] & 0x1;
+      /**
+       * This should shift value of register y right by one bit and assign it
+       * to register x. However it seems like most implementations actually
+       * just shift the register x right by one bit and store the least
+       * significant bit prior to the sift in register VF.
+       */
+      vm.registers[0xf] = vm.registers[register1] & 0x1;
+      vm.registers[register1] = vm.registers[register1] >> 1;
       break;
     case 0x7: //Subtract register x from register y and assign to register x
       sub = vm.registers[register2] - vm.registers[register1];
@@ -205,10 +209,14 @@ export function betweenRegisters(opcode: number, vm: VM) {
       vm.registers[0xf] = sub < 0 ? 0 : 1;
       break;
     case 0xe:
-      //Shift value of register y left by one bit and assign it to register x
-      //Store the most significant bit of register y in VF
-      vm.registers[register1] = (vm.registers[register2] << 1) & 0xff;
-      vm.registers[0xf] = vm.registers[register2] >> 7;
+      /**
+       * This should shift value of register y left by one bit and assign it
+       * to register x. However it seems like most implementations actually
+       * just shift the register y left by one bit and store the most
+       * significant bit prior to the shift in register VF.
+       */
+      vm.registers[0xf] = vm.registers[register1] >> 7;
+      vm.registers[register1] = (vm.registers[register1] << 1) & 0xff;
       break;
     default:
       throw new OpcodeError(

test/vm.ts

@@ -195,26 +195,36 @@ describe("Virtual machine", () => {
       expect(vm.registers[0xa]).to.equal(0xff);
       expect(vm.registers[0xf]).to.equal(0x0);
     });
-    it("should shift register right by one bit and assign it to another register", () => {
-      const vm = initializeVm([0x6bee, 0x8ab6, 0x6bff, 0x8ab6]);
-      //Shift value register VB to the right by 1 bit and and store it in register
-      //VA - set register VF to the least significant bit before the shift
+    it("should shift register right by one bit", () => {
+      const vm = initializeVm([
+        0x6bee, //Store 0xEE in register VB (least significant bit is 0)
+        0x8bb6, //Shift register VB right by one bit
+        0x6bff, //Store 0xFF in register VB (least significant bit is 1)
+        0x8bb6 //Shift register VB right by one bit
+      ]);
+      //Register VB should be shifted right and register VF should be 0
       times(2, () => vm.next());
-      expect(vm.registers[0xa]).to.equal(0x77);
+      expect(vm.registers[0xb]).to.equal(0x77);
       expect(vm.registers[0xf]).to.equal(0x0);
+      //Register VB should be shifted right and register VF should be 1
       times(2, () => vm.next());
-      expect(vm.registers[0xa]).to.equal(0x7f);
+      expect(vm.registers[0xb]).to.equal(0x7f);
       expect(vm.registers[0xf]).to.equal(0x1);
     });
-    it("should shift register left by one bit and assign it to another register", () => {
-      const vm = initializeVm([0x6bee, 0x8abe, 0x6b7f, 0x8abe]);
-      //Shift value register VB to the left by 1 bit and and store it in register
-      //VA - set register VF to the most significant bit before the shift
+    it("should shift register left by one bit", () => {
+      const vm = initializeVm([
+        0x6bee, //Store 0xEE in register VB (most significant bit is 1)
+        0x8bbe, //Shift register VB left by one bit
+        0x6b7f, //Store 0x7F in register VB (most significant bit is 0)
+        0x8bbe //Shift register VB left by one bit
+      ]);
+      //Register VB should be shifted left and register VF should be 1
       times(2, () => vm.next());
-      expect(vm.registers[0xa]).to.equal(0xdc);
+      expect(vm.registers[0xb]).to.equal(0xdc);
       expect(vm.registers[0xf]).to.equal(0x1);
+      //Register VB should be shifted left and register VF should be 0
       times(2, () => vm.next());
-      expect(vm.registers[0xa]).to.equal(0xfe);
+      expect(vm.registers[0xb]).to.equal(0xfe);
       expect(vm.registers[0xf]).to.equal(0x0);
     });
   });