09-23-25 Lab 8

Hello world! in assembly:

section .data
    msg db "Hello, world!", 10 ; Our string, including a newline character (ASCII 10)
    len equ $ - msg           ; Length of the string

section .text
    global _start             ; Entry point for the linker

_start:
    ; Write "Hello, world!" to standard output
    mov rax, 1                ; syscall number for sys_write (1)
    mov rdi, 1                ; File descriptor for standard output (1)
    mov rsi, msg              ; Address of the string to write
    mov rdx, len              ; Length of the string
    syscall                   ; Invoke the kernel

    ; Exit the program
    mov rax, 60               ; syscall number for sys_exit (60)
    mov rdi, 0                ; Exit status (0 for success)
    syscall                   ; Invoke the kernel

Explanation:

• .section .data: 
  This section declares initialized data.

  • msg db "Hello, world!", 10: Defines a byte string named msg containing "Hello, world!" followed by a newline character (ASCII 10).
  • len equ $ - msg: Defines a constant len which calculates the length of the msg string. $ represents the current address.

• .section .text: 
  This section contains the executable code.

  • global _start: Makes the _start label visible to the linker, marking it as the program's entry point.

• _start:: 
  The program's entry point.

  • mov rax, 1: Loads the system call number for sys_write (which is 1) into the rax register.
  • mov rdi, 1: Loads the file descriptor for standard output (which is 1) into the rdi register.
  • mov rsi, msg: Loads the memory address of our msg string into the rsi register.
  • mov rdx, len: Loads the length of our msg string into the rdx register.
  • syscall: Executes the system call, using the values in rax, rdi, rsi, and rdx as arguments.
  • mov rax, 60: Loads the system call number for sys_exit (which is 60) into the rax register.
  • mov rdi, 0: Loads the exit status (0 for success) into the rdi register.
  • syscall: Executes the system call to terminate the program.

Install NASM asembler:

sudo apt-get install nasm

Save the code as hello.asm and Assemble the code using NASM.

nasm -f elf64 -o hello.o hello.asm

Link the object file to create an executable:

ld -o hello hello.o

Run the executable

./hello

Jump table

...

Jump example code

With bug:

section .txt
global _start

_start:
	mov eax, 3
	mov ebx, 2
	cmp eax, ebx
	JL lesser

lesser:
	mov ecx, 1
	INT 80h
	
# What is the bug here?
	# We want to go to the next instruction
	# only if one is less than the other one.

Fixed:

section .txt
global _start

_start:
	mov eax, 3
	mov ebx, 2
	cmp eax, ebx
	JL lesser
	JMP end

lesser:
	mov ecx, 1
	INT 80h

end:
	INT 80h

Use gdb to debug assembly

Break at my _start

(gdb) break _start_

Go to the next assembly instruction

(gdb) stepi

• Try changing numbers in the code, comparing them and see on gdb how these change as well.