Writing Functions in Assembler Code

We give a minimalistic example on how a function written in assembly code can be called from a C program:

• The C program calling an external function:

  session18/example/test_factorial.c

  #include <stdio.h>
  #include <stddef.h>     // for size_t
  #include <stdint.h>     // for uint64_t
  
  // forward declaration
  uint64_t
  factorial(uint64_t n);
  
  int
  main()
  {
      size_t n;
  
      printf("n = ");
      scanf("%zu", &n);
  
      n = factorial(n);
  
      printf("n! = %zu\n", n);
  }
  

• The function written in assembly code

  session18/example/factorial.s

  #  uint64_t
  #  factorial(uint64_t n);
  
  #  Note: n is passed in register %rdi
  #        the retrun value is passed in register %rax
  
  .globl factorial
  factorial:
  
          movq    $1,     %rax
          jmp     .check
  .loop:
          imulq   %rdi,   %rax
          decq    %rdi
  
  .check:
          cmpq    $1,   %rdi
          jg      .loop
  
  
          retq
  

Exercise

Compile, link an run the code.

View document source

© Michael C. Lehn