16.4: Assembly Language Example

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An example assembly language program to read and display the command line arguments is included for reference. This example simply reads and displays the command line arguments.

;  Command Line Arguments Example
; -------------------------------------------------------

section .data 

; -----
; Define standard constants. 
LF           equ     10                ; line feed
NULL         equ     0                 ; end of string
TRUE         equ     1
FALSE        equ     0 
EXIT_SUCCESS equ     0                 ; success code

STDIN        equ     0                 ; standard input
STDOUT       equ     1                 ; standard output
STDERR       equ     2                 ; standard error

SYS_read     equ     0                 ; read
SYS_write    equ     1                 ; write
SYS_open     equ     2                 ; file open
SYS_close    equ     3                 ; file close
SYS_fork     equ     57                ; fork
SYS_exit     equ     60                ; terminate
SYS_creat    equ     85                ; file open/create
SYS_time     equ     201               ; get time

; -----
;  Variables for main.

newLine db LF, NULL
; ------------------------------------------------------
section .text global main main:
; -----
;  Get command line arguments and echo to screen.
;  Based on the standard calling convention,
;    rdi = argc (argument count)
;    rsi = argv (starting address of argument vector)

    mov    r12, rdi            ; save for later use...
    mov    r13, rsi           

; -----
;  Simple loop to display each argument to the screen.
;  Each argument is a NULL terminated string, so can just
;  print directly.

printArguments:
    mov     rdi, newLine 
    call    printString     
    
    mov     rbx, 0
printLoop:
    mov     rdi, qword [r13+rbx*8] 
    call    printString

    mov     rdi, newLine call printString
    inc     rbx
    cmp     rbx, r12 
    jl      printLoop
    
; -----
;  Example program done.

exampleDone:
    mov     rax, SYS_exit 
    mov     rdi, EXIT_SUCCESS 
    syscall

; **********************************************************
;  Generic procedure to display a string to the screen.
;  String must be NULL terminated.
;  Algorithm:
;  Count characters in string (excluding NULL)
;  Use syscall to output characters 
;  Arguments:
;      1) address, string
; Returns:
;      nothing

global printString printString:
    push    rbp
    mov     rbp, rsp 
    push    rbx
; -----
;  Count characters in string.

    mov     rbx, rdi
    mov     rdx, 0 
strCountLoop:
    cmp     byte [rbx], NULL 
    je      strCountDone   
    inc     rdx
    inc     rbx
    jmp     strCountLoop
strCountDone:

    cmp     rdx, 0 
    je      prtDone

; -----
;  Call OS to output string.
  
    mov    rax, SYS_write                ; code for write ()
    mov    rsi, rdi                      ; addr of characters
    mov    edi, STDOUT                   ; file descriptor
                                         ; count set above
    syscall                              ; system call
    
; -----
;  String printed, return to calling routine.

prtDone:
    pop     rbx
    pop     rbp 
    ret
    
; *******************************************************

The printString() function is repeated in this example and is unchanged from the previous examples.

It must be noted that in order for this to work, the program should be assembled as usual but linked with the GNU C compiler, either GCC or G++.

For example, assuming this example program is named cmdLine.asm, the assembly and linking would be as follows:

       yasm -g dwarf2 -f elf64 cmdLine.asm -l cmdLine.lst
       gcc -g -o cmdLine cmdLine.o

Note, Ubuntu 18 will require the no-pie option on the gcc command as shown:

       gcc -g -no-pie -o cmdLine cmdLine.o

If the standard linker is used, the arguments will not be passed in the correct manner.