Definitions

   The factorial of   0   (zero)   is defined as being   1   (unity).
   The   Factorial Function   of a positive integer,    n,    is defined as the product of the sequence:
                 n,   n-1,   n-2,   ...   1 



Task

Write a function to return the factorial of a number.

Solutions can be iterative or recursive.

Support for trapping negative    n    errors is optional.



Related task

   Primorial numbers


        LDA  x
        BRZ  done_i   ; 0! = 1
 
        STA  i
 
loop_i: LDA  fact
        STA  n
 
        LDA  i
        SUB  one
        BRZ  done_i
 
        STA  j
 
loop_j: LDA  fact
        ADD  n
        STA  fact
 
        LDA  j
        SUB  one
        BRZ  done_j
 
        STA  j
        JMP  loop_j
 
done_j: LDA  i
        SUB  one
        STA  i
 
        JMP  loop_i
 
done_i: LDA  fact
        STP
 
one:         1
 
fact:        1
 
i:           0
j:           0
n:           0
 
x:           5