Add call/ret/push/pop instructions

* Call/ret/push/pop are implemented and appear to be working
* Call/ret/push/pop is specified in the 0x2000 block, replacing the
  mov instruction
* Mov instruction is now specified in the 0x3000 block

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

vm.md

@@ -181,10 +181,33 @@ wrapping around to 0.
     * Opcode: 0x1004
     * Params: Source
 
+## Functions
+
+* Call
+    * Opcode: 0x2000
+    * Params: Source
+    * When this instruction is executed, these actions occur:
+        * Push the current stack frame pointer
+        * Push the IP of the next instruction
+        * Update the IP (i.e., jump) to the value at the given source.
+* Ret
+    * Opcode: 0x2001
+    * When this instruction is executed, these actions occur:
+        * Update the stack pointer to the current frame pointer + 16.
+        * Pop the IP of the next instruction.
+        * Pop the old stack frame.
+        * Restore the last three values in an undefined order
+* Push
+    * Opcode: 0x2002
+    * Params: Source
+* Pop
+    * Opcode: 0x2003
+    * Params: Dest
+
 ## Data movement
 
 * Mov
-    * Opcode: 0x2000
+    * Opcode: 0x3000
 
 ## Miscellaneous