How to compile using nasm on MacOSX

Question

I am trying to compile and link my first program on Assembler. I try to compile the following code:

; %include "stud_io.inc"    
global _main     

section .text
_main: 
    xor eax, eax
again:
    ; PRINT "Hello"
    ; PUTCHAR 10
    inc eax     
    cmp eax, 5
    jl again

Below the console command for compiling and linking a program:

-bash-3.2$ nasm -f macho main.asm -o main.o  && ld -e _main -macosx_version_min 10.8 -arch x86_64 main.o

But the result is:

ld: warning: ignoring file main.o, file was built for i386 which is not the architecture being linked (x86_64): main.o
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     -u command line option
ld: symbol(s) not found for architecture x86_64

I think its necessary to compile the main.asm file for x86_64.. How to compile programs for my system properly?

Solution 1

I would recommend first updating your NASM.

After that, try running this:

nasm -f macho64 main.asm -o main.o  && ld -e _main -macosx_version_min 10.8 -arch x86_64 main.o -lSystem

Notice that the new command adds JasonD's suggestion above (macho64), but also adds the -lSystem to the ld command to stop ld from throwing following error:

ld: dynamic main executables must link with libSystem.dylib for architecture x86_64

Solution 2

I noticed that most examples show standalone assembly programs, but it is perhaps more common for assembly to be called from C. I created a simple C program that uses a minimal nasm-assembled function like this:

extern unsigned cpuid(unsigned n);

/* ... */
        unsigned n = cpuid(1);

The assembly looks like this:

section .text
    global _cpuid

_cpuid:
    push rbp
    mov rbp, rsp
    mov rax, rdi
    cpuid
    mov rax, rcx
    leave
    ret

You can see the whole thing, including nasm CLI options in the makefile, here:

https://github.com/ecashin/low/tree/master/cpuid

It does something mildly useful by printing out the availability of some CPU-specific features. (But it does that by using CPUID without checking whether it's available. If the CPU is Intel and newer than an i486, though, that's fine.)

The example is tested on Mac OS X Snow Leopard with the nasm from the ports collection. Removing the underscore prefix is the only change necessary for porting to Linux x86_64.

Comments

[RageD]

That version of nasm is very outdated. Look at nasm.us/pub/nasm/releasebuilds/2.10

[RageD]

The segfault is a problem with the code execution. After the jl instruction, add a ret instruction (since you are using a main). Note that if you are using pure x86 assembly, you need to exit by using the exit system call in int 0x80 (but not in this case). Remember that the PC counter needs to know where to go to continue execution when your code is done.

[Ed Cashin]

Also, I did need to update NASM as the other comments here say before I could use -fmacho64.

[Elshan]

ld -lSystem my.o -e _main -macosx_version_min 10.14 -o exit Result - > cannot execute binary file