Virtual Machine Instructions

1. Machine instructions are C assignment statements restricted to the following grammar:

   instruction $\rightarrow$ unlabeled_instruction $\vert$ labeled_instruction
   labeled_instruction $\rightarrow$ label : unlabeled_instruction
   label $\rightarrow$ identifier
   unlabeled_instruction $\rightarrow$ lhs = rhs $\vert$ push(pushargument) $\vert$ if lhs goto label
   unlabeled_instruction $\rightarrow$ goto label $\vert$ pop() $\vert$ procname() $\vert$ return $\vert$ putint(putintargument)
   pushargument $\rightarrow$ lhs $\vert$ smallterm
   lhs $\rightarrow$ stack[term] $\vert$ term $\vert$ register
   term $\rightarrow$ stack[smallterm]
   smallterm $\rightarrow$ register + const $\vert$ register - const $\vert$ register
   register $\rightarrow$ R$_i$ $\vert$ BP $\vert$ SP
   const $\rightarrow$ integer
   rhs $\rightarrow$ const $\vert$ lhs $\vert$ register op lhs $\vert$ register op const $\vert$ pop() $\vert$ getint()
   putintargument $\rightarrow$ rhs $\vert$ smallterm
   op $\rightarrow$ + $\vert$ - $\vert$ * $\vert$ % $\vert$ / $\vert$ < $\vert$ =< $\vert$ > $\vert$ >= $\vert$ == $\vert$ !=

   There is a restriction that if the rhs is register op lhs or register op const, then lhs must be the same register name as in regsiter.

2. Instructions are combined using ; and are grouped in parameterless procedures. There must be one parameterless function func for each function func in the source program. Note that this function cannot return any value.

3. push , pop , getint and putint are defined as:

   void push(int element)
   {
       stack[++SP]=element;
       return;
   }
   int pop()
   {
       return(stack[SP--]);
   }
   int getint()
   {
       int a;
       scanf("%d",&a);
       return a;
   }
   void putint(int a)
   {
       printf("%d",a);
       return;
   }