diff --git a/creature.go b/creature.go
index 7e6b945..63f1b29 100644
--- a/creature.go
+++ b/creature.go
@@ -8,13 +8,24 @@ type Machine struct {
 	pointer int
 }
 
+// Execute starts execution at the address specified by offset. The machine will
+// run until a HALT instruction is encountered, or the end of program memory is
+// reached.
 func (machine *Machine) Execute (offset int) {
 	machine.counter = offset
 
 	for {
-		switch machine.Program[machine.counter] {
+		switch machine.instruction() {
 		case 0x0:
+			// PUSH
+			// push the next word in program memory onto the stack
+			machine.counter ++
+			machine.Push(machine.instruction())
+			
 		case 0x1:
+			// POP
+			// pop the top word off of the stack, and discard it
+			machine.Pop()
 		case 0x2:
 		case 0x3:
 		case 0x4:
@@ -34,6 +45,12 @@ func (machine *Machine) Execute (offset int) {
 	}
 }
 
+// Instruction returns the current instruction in program memory.
+func (machine *Machine) instruction () (instruction int) {
+	instruction = machine.Program[machine.counter]
+	return
+}
+
 // reallocateStack changes the size of the stack to something reasonable. This
 // should be called then the stack pointer is really small compared to the
 // actual stack size, or the stack pointer is bigger than the stack.