allocate-registers a, b, one, factorial-product, end

   li a, 1
   read b
   li one, 1
   li factorial-product, factorial-product-label
   li end, end-label

factorial-product-label:
   ;; computes a * b! into a and then jumps to end
   ;;  provided that b is a non-negative integer;
   ;;  assumes that the register named one contains 1 and
   ;;  the factorial-product register contains this address;
   ;;  may also change the b register's contents
   jeqz b, end   ; if b = 0, a * b! is already in a

   mul a, a, b   ; otherwise, we can put a * b into a
   sub b, b, one ; and b - 1 into b, and start the
   j factorial-product            ; iteration over

end-label:
   write a
   halt