.TITLE   Permutation
;  Calculation of P(N,R) - the number of permutations
;  of N things taken R at a time using a function for
;  evaluation of Factorials

;  Method:  Calculate P(N,R) using N!/(N - R)!

         .DISABLE GLOBAL

;  Uses external function Factorial
         .EXTERNAL Factorial

;  Program Variables:
N:       .LONG    5
R:       .LONG    2
PNR:     .BLKL    1                ; P(N,R)
NMinusR: .BLKL    1                ; N - R

;  The main program:
         .ENTRY   Permutations, 0

;     R3 <-- N!
Start:   PUSHAL   N                ; Push address of N
         CALLS    #1, Factorial    ; Calculate N!
         MOVL     R0, R3           ; Save N!

;     R0 <-- (N - R)!
         SUBL3    R, N, NMinusR    ; Calculate N - R
         PUSHAL   NMinusR          ; Push address N - R
         CALLS    #1, Factorial    ; Calculate (N - R)!

;     P(N,R) <-- quotient
         DIVL3    R0, R3, PNR

Finish:  $EXIT_S

         .END      Permutations