Stacks

Often implemented as an ADT
#define STACKMAX 32
static int opstack[STACKMAX];
static int opsp;

PushOp(op)
   int op;
{
  if (opsp==STACKMAX) {strcpy(dispstr,"stack error");  entered=3;}
  else opstack[opsp++]=op;
}

int PopOp()
{
  if (opsp==0) {
      strcpy(dispstr,"stack error");
      entered=3;
      return(kNOP);
  } else
    return(opstack[--opsp]);
}

int isopempty()
{
  return( opsp ? 0 : 1 );
}
Also implemented in-line:
    de_stack[--stack_top] = '0' + c % 8;
    de_stack[--stack_top] = '0' + (c / 8) % 8;
    de_stack[--stack_top] = '0' + c / 64;

Problem with overflow-underflow checking.