Implemented memory and arithmetic instructions

creature.go

@@ -28,11 +28,39 @@ func (machine *Machine) Execute (offset int) {
 			machine.Pop()
 			
 		case 0x2:
+			// LOAD
+			// push the word at an address onto the stack
+			machine.Push(machine.Peek(machine.Pop()))
+		
 		case 0x3:
+			// STOR
+			// store a word at an address
+			machine.Poke(machine.Pop(), machine.Pop())
+		
 		case 0x4:
+			// ADD
+			// adds the last two words on the stack
+			machine.Push(machine.Pop() + machine.Pop())
+			
 		case 0x5:
+			// SUB
+			// subtracts the last two words on the stack
+			right := machine.Pop()
+			left  := machine.Pop()
+			machine.Push(left - right)
+			
 		case 0x6:
+			// MUL
+			// multiplies the last two words on the stack
+			machine.Push(machine.Pop() * machine.Pop())
+			
 		case 0x7:
+			// DIV
+			// divides the last two words on the stack
+			right := machine.Pop()
+			left  := machine.Pop()
+			machine.Push(left / right)
+			
 		case 0x8:
 		case 0x9:
 		case 0xa: