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copied!<p>I was confused in the same case on FreeBSD 9.0/amd64. What I did is (I used nasm for assembler):</p> <pre><code>$ cat foo.asm global _start _start: mov rax, 4 ; write mov rdi, 1 ; stdout mov rsi, rsp ; address mov rdx, 16 ; 16bytes syscall mov rax, 1 ; exit syscall $ nasm -f elf64 foo.asm && ld -o foo foo.o $ ./foo | hd 00000000 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 |................| 00000010 $ ./foo 2 | hd 00000000 02 00 00 00 00 00 00 00 b8 dc ff ff ff 7f 00 00 |................| 00000010 $ ./foo 2 3 | hd 00000000 00 00 00 00 00 00 00 00 03 00 00 00 00 00 00 00 |................| 00000010 $ ./foo 2 3 4 | hd 00000000 00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 |................| 00000010 $ ./foo 2 3 4 5 | hd 00000000 05 00 00 00 00 00 00 00 b0 dc ff ff ff 7f 00 00 |................| 00000010 </code></pre> <p>I expected that argc was at rsp, but it was not.</p> <p>I guessed that the kernel (the image activator) sets registers. I searched the source tree, I found the following code in /usr/src/sys/amd64/amd64/machdep.c (exec_setregs).</p> <pre><code> regs->tf_rsp = ((stack - 8) & ~0xFul) + 8; regs->tf_rdi = stack; /* argv */ </code></pre> <p>These lines look saying that rsp is aligned, actual data are at rdi. I changed my code, and I got expected results.</p> <pre><code>$ cat foo.asm global _start _start: push rdi mov rax, 4 ; write mov rdi, 1 ; stdout pop rsi mov rdx, 16 ; 16bytes syscall mov rax, 1 ; exit syscall $ nasm -f elf64 foo.asm && ld -o foo foo.o $ ./foo | hd 00000000 01 00 00 00 00 00 00 00 b0 dc ff ff ff 7f 00 00 |................| 00000010 $ ./foo 2 | hd 00000000 02 00 00 00 00 00 00 00 a8 dc ff ff ff 7f 00 00 |................| 00000010 $ ./foo 2 3 | hd 00000000 03 00 00 00 00 00 00 00 a8 dc ff ff ff 7f 00 00 |................| 00000010 $ ./foo 2 3 4 | hd 00000000 04 00 00 00 00 00 00 00 a8 dc ff ff ff 7f 00 00 |................| 00000010 $ ./foo 2 3 4 5 | hd 00000000 05 00 00 00 00 00 00 00 a8 dc ff ff ff 7f 00 00 |................| 00000010 </code></pre> <p>Can you try rdi?</p>

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