Other assembly programming with nasm

[any_name_you_wish]

Hello,

how to compile a simple program (written in assembly) with nasm?

SECTION .data
msg     db      'Hello World!', 0Ah
 
SECTION .text
global  _start
 
_start:
 
    mov     edx, 13
    mov     ecx, msg
    mov     ebx, 1
    mov     eax, 4
    int     80h
 
    mov     ebx, 0      ; return 0 status on exit - 'No Errors'
    mov     eax, 1      ; invoke SYS_EXIT (kernel opcode 1)
    int     80h

https://asmtutor.com/#lesson1

My commands :

nasm -f elf hello.s
ld -m elf_i386_fbsd -o hello -s hello.o
./hello

There are no errors and no output (no "Hello World!").

 

[_martin]

Have a look on FreeBSD 's calling conventions. You need to push on stack. Beware of an extra push of dword with direct int 0x80.
truss(1) and under i386 devel/strace are good tools to use to debug stuff. Also single stepping in gdb(1) is good idea to see what's happening.

 

[any_name_you_wish]

syscalls (opcodes) are the same like in tutorial: 4 for write and 1 for exit.

 

[_martin]

Do peek at calling conventions too. Modified example for FreeBSD:

section .data
    msg     db      'Hello World!', 0Ah

section .text
    global _start

_start:
    push dword 13
    push dword msg
    push dword 1
    mov eax, 4
    push eax
    int 0x80

    push 0
    mov eax, 1
    push eax
    int 0x80

 

[any_name_you_wish]

Modified example

It works.

And to run my code I have to install "linux emulation", right? ;-)
(or to work with "push")

 

[any_name_you_wish]

Modified example

By the way: would this code run on Linux?

 

[_martin]

FreeBSD provides kernel module - linux(4). You'd have to have it loaded and your binary brandelf(1) it. You should stick to FreeBSD native ABI.
No, this modified code would not yield the result you're hoping for on Linux. I don't want to go too much into the details here but do have a look on those calling conventions and ABI.

 

[any_name_you_wish]

Ok, got it.
Thank you!

 

[Alain De Vos]

Exercise:
-1.Modify the code for 64-bit
-2.Modify the code for libc

 

[a6h]

I've posted a 101 on how to use (Hello World style) nasm, fasm, as, gas32 and __asm__ in C. All have has been tested on a FreeBSD/amd64 machine.

Other - Difference between "as" and "ld" on BSD vs Linux (assembly)?

I started reading "Programming from the Ground Up", a free programming book that starts new programmers with assembly, and the first two example programs that I've tried so far didn't run correctly on FreeBSD (but worked on Fedora). These simple programs store a calculation in an exit code...

forums.freebsd.org

Let me know, if you have any questions, or any ambiguity about that post.

 

[Alain De Vos]

Vigole, i tried some 64bit code but it gave bus error.
Then i found this which runs ok ,

Solved - Assembly - simple Hello World

Hi, I'm beginning assembly on FreeBSD and tried a simple HelloWorld. I'm using 2 separate files, one for some macro and defines, one for the actual code (looks a long post but hang on, it's just hello world =D ) Here is the macro file : ; ; system.inc : aliases definitions (defines & macros)...

forums.freebsd.org

 

[a6h]

Vigole, i tried some 64bit code but it gave bus error.

Because I don't know exactly, what error message you've received, I have to guess that you probably missed the sys.exit call, which is necessary at the end of .s file, or you have a typo -- or even missing part, in the %macro definition of the sys.exit in the .inc file. Both can produce the Bus Error (core dumped).