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copied!<p>First, you are using old 32-bit Linux kernel calling convention on Mac OS X - this absolutely doesn't work.</p> <p>Second, syscalls in Mac OS X are structured in a different way - they all have a leading class identifier <em>and</em> a syscall number. The class can be Mach, BSD or something else (see <a href="http://www.opensource.apple.com/source/xnu/xnu-1699.26.8/osfmk/mach/i386/syscall_sw.h" rel="noreferrer">here</a> in the XNU source) and is shifted 24 bits to the left. Normal BSD syscalls have class <code>2</code> and thus begin from <code>0x2000000</code>. Syscalls in class <code>0</code> are <em>invalid</em>.</p> <p>As per §A.2.1 of the <a href="http://www.x86-64.org/documentation/abi.pdf" rel="noreferrer">SysV AMD64 ABI</a>, also followed by Mac OS X, syscall id (together with its class on XNU!) goes to <code>%rax</code> (or to <code>%eax</code> as the high 32 bits are unused on XNU). The fist argument goes in <code>%rdi</code>. Next goes to <code>%rsi</code>. And so on. <code>%rcx</code> is used by the kernel and its value is destroyed and that's why all functions in <code>libc.dyld</code> save it into <code>%r10</code> before making syscalls (similarly to the <code>kernel_trap</code> macro from <code>syscall_sw.h</code>).</p> <p>Third, code sections in Mach-O binaries are called <code>__text</code> and not <code>.text</code> as in Linux ELF and also reside in the <code>__TEXT</code> segment, collectively referred as <code>(__TEXT,__text)</code> (<code>nasm</code> automatically translates <code>.text</code> as appropriate if Mach-O is selected as target object type) - see the <a href="https://developer.apple.com/library/mac/#documentation/DeveloperTools/Conceptual/MachORuntime/Reference/reference.html" rel="noreferrer">Mac OS X ABI Mach-O File Format Reference</a>. Even if you get the assembly instructions right, putting them in the wrong segment/section leads to bus error. You can either use the <code>.section __TEXT,__text</code> directive (see <a href="https://developer.apple.com/library/mac/#documentation/developertools/Reference/Assembler/040-Assembler_Directives/asm_directives.html" rel="noreferrer">here</a> for directive syntax) or you can also use the (simpler) <code>.text</code> directive, or you can drop it altogether since it is assumed if no <code>-n</code> option was supplied to <code>as</code> (see the manpage of <code>as</code>).</p> <p>Fourth, the default entry point for the Mach-O <code>ld</code> is called <code>start</code> (although, as you've already figured it out, it can be changed via the <code>-e</code> linker option).</p> <p>Given all the above you should modify your assembler source to read as follows:</p> <pre><code>; You could also add one of the following directives for completeness ; .text ; or ; .section __TEXT,__text .globl start start: movl $0x2000001, %eax movl $32, %edi syscall </code></pre> <p>Here it is, working as expected:</p> <pre><code>$ as -o exit.o exit.s; ld -o exit exit.o $ ./exit; echo $? 32 </code></pre>

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