      if (Pop()==1) pc=adr;
HALT      End of the program  Halt;
NEG       Negation            Push(-Pop());
READ l,a  Read integer        stack[Frame(l)+a]=Read();
WRITE     Write integer       Writeln(Pop());
</PRE>

<P>The function <I>Frame(l)</I> returns the base address of the stack frame which
is (statically) <I>l</I> levels up in the stack. <I>l</I>=0 means the current frame,
<I>l</I>=1 means the statically surrounding frame, etc. The main program is also
considered as a procedure with a stack frame. 