Assembly Language Programming

1. Directives

.text       # codes

.data       # data section of data having initial value
# type declaration required

.bss        # data section of data without initial value
# save memory; no type declaration required

# .p2align expr1, expr2, expr3
# align to 2^expr1, padding with expr2, max padding number of bytes expr3
.p2align 4, , 15
# If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all.

.section .rodata # read-only code section
LC0:    # local
    .ascii  "Hello World\12\0"

Example:

.globl  _c

.data   # data section
.align 4
_c: # start address symbol
    .long 1
# int c = 1;

gcc -O2 \
-mpreferred-stack-boundary=2 \  # align to 2^2=4
-fomit-frame-pointer \  # omit the frame pointer %rbp
helloworld.c -S

Frame Pointer:

Memory Layout:

2. System call

as -o myobj.o hello.s
# -gstabs with debugging info

ld myobj.o -o myobj

.data   # data section
msg:
    .ascii "Hello World\n"
    len = . - msg   # . means the current location
.text   # code section
.globl _start   # entry point
_start:
    movq    $len, %rdx  # 3th arg. of sys. call, how many to write
    movq    $msg, %rsi  # 2th arg. of sys. call, starting address of what to write
    movq    $1, %rax    # code of "write"
    movq    $1, %rdi    # write to 1(stdout)
    # int   $0x80       # interrupt instruction in i386
    syscall

    movq    $60, %rax   # code of "exit"
    movq    $0, %rdi    # arg1 of exit, 0, exit status
    syscall

When executing int $0x80,

• %rax stores the function/service code of a system call.
• %rdi, %rsi, %rdx, %r10, %r8, %r9 are arguments.
• More arguments are shored in a continuous area in memory which is pointed by %rbx.

After calling, %eax stores the return value.

3. Command Line Arguments

int argc, char* argv[] are pushed on the the stack when the program starts.

.text
.globl  _start
_start:
    popq %rsi   # argc
vnext:
    popq %rsi   # argv[?]
    testq   %rsi, %rsi
    jz  exit    # jump if zero
    movq    %rsi, %rbx
    xorq    %rdx, %rdx  # get 0
strlen:
    movb    (%rbx), %al # get the last 8-bit as a byte
    incq    %rdx
    incq    %rbx
    testb   %al, %al
    jnz strlen
    movb    $10, -1(%rbx)   # 10 is a new line
    movq    $1, %rax    # system call
    movq    $1, %rdi # write to stdout
    syscall
    jmp vnext
exit:
    movq    $60, %rax
    movq    $0, %rdi
    syscall

4. Call Functions in C Library

.section    .rodata
.LC0:
    .string "hello\n"

.text
.globl  _start

_start:
    movl    $.LC0, %edi
    call puts
    movl $0, %edi
    call exit

as -o hello.o hello.s
ld -lc -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o hello hello.o

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Last update: September 7, 2021

Authors: Co1lin