Fibonacci sequence by recursive algorithm fail

[compsystems]

Code:

#CAS
    nfibo_cas(n):=
    begin
        // /!\ n = N+
        //    version 0.0.3 April 22
        if n≤1 then
            return(n);
            else
            return(nfibo_cas(n-1)+nfibo_cas(n-2));
        end;
    end;
#end

nfibo_cas(28) → 317811
nfibo_cas(30) → BUSY → 832040
nfibo_cas(100) → running non recursive evaluator

What means the above output?

nfibo_cas(100) → ¨ERROR MEMORY¨ (ti68k calcs)