Unaligned access

Misalignment is not an error (only incurs a performance penalty) on x86 processors except for a few new instructions added in recent years. MOVDQA, for example, is an SSE2 instruction requiring alignment on 16-byte boundaries.

Textbooks have normally taught us we get a bus error if a CPU which disallows unaligned access actually encounters one.

But we observe a Linux process passing misaligned addresses to MOVDQA receives SIGSEGV (segmentation fault) instead of SIGBUS (bus error), on both ia32 and x86-64.

laptop /tmp $ cat a.c
int main ()
{
    char X[32];
    asm ("pxor %%xmm0,%%xmm0; movdqa %%xmm0,%0" : "=m"(X[1]) :: "xmm0");
    return 0;
}
laptop /tmp $ gcc -msse2 a.c
laptop /tmp $ ./a.out
Segmentation fault
laptop /tmp $ kill -l $?
SEGV

x86-64 (and ia32 beginning 80486SX) supports disallowing any misaligned access*. In that case, a normal instruction raises SIGBUS, but instructions which inherently requires alignment (e.g. MOVDQA) still raises SIGSEGV. It’s not so consistent.

* It is normally disabled. To enable it, set the AC bit in FLAGS:

pushf
or $0x40000,(%esp) (or %rsp on x86-64)
popf

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Tags: assembly, Linux, segfault, x86-64