16.4: Assembly Language Example
- Page ID
- 19963
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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.