Recursive Call

         . . . . . .

recurse:                          # else
                                  #   return n*fact(n-1);
         sub     $a0,$s1,1        #     argument0 = n-1
                       
                                  # subroutine call
                                  #   1. No T registers to push
                                  #   2. Argument is in $a0 
         jal     fact             #   3. Jump and link to subroutine

         mul     $v0,$v0,$s1      # n*fact(n-1)

epilog:                           # epilog
                                  #   1. Return value is already in $v0        
         . . . . . .
         jr      $ra              #

Recursion has been implemented by using: (1) the normal machine operations of sequential execution, testing, and branching, and (2) the run-time stack.

This is (yet another) example of a new level of abstraction being build upon a foundation level. Have I mentioned that this is one of the most stunningly important ideas of Computer Science?