易百教程

section    .text
   global _start         ;must be declared for using gcc

_start:                  ;tell linker entry point

   mov bx, 3             ;for calculating factorial 3
   call  proc_fact
   add   ax, 30h
   mov  [fact], ax

   mov      edx,len        ;message length
   mov      ecx,msg        ;message to write
   mov      ebx,1          ;file descriptor (stdout)
   mov      eax,4          ;system call number (sys_write)
   int      0x80           ;call kernel

   mov   edx,1            ;message length
   mov      ecx,fact       ;message to write
   mov      ebx,1          ;file descriptor (stdout)
   mov      eax,4          ;system call number (sys_write)
   int      0x80           ;call kernel

   mov      eax,1          ;system call number (sys_exit)
   int      0x80           ;call kernel

proc_fact:
   cmp   bl, 1
   jg    do_calculation
   mov   ax, 1
   ret

do_calculation:
   dec   bl
   call  proc_fact
   inc   bl
   mul   bl        ;ax = al * bl
   ret

section    .data
msg db 'Factorial 3 is:',0xa    
len equ $ - msg            

section .bss
fact resb 1