Legal Limit

22/06/2015
/* Push x onto the execution stack.  This used to be #define PUSH(x)
   (*++stackp = (x)) This oddity is necessary because Alliant can't be
   bothered to compile the preincrement operator properly, as of 4/91.
   -JimB */

#define PUSH(x) (top++, *top = (x))

Source.

Are you sure 21 years is old enough?