2Memory alignment
   5Too many problems popped up because of unnoticed misaligned memory access in
   6kernel code lately.  Therefore the alignment fixup is now unconditionally
   7configured in for SA11x0 based targets.  According to Alan Cox, this is a
   8bad idea to configure it out, but Russell King has some good reasons for
   9doing so on some f***ed up ARM architectures like the EBSA110.  However
  10this is not the case on many design I'm aware of, like all SA11x0 based
  13Of course this is a bad idea to rely on the alignment trap to perform
  14unaligned memory access in general.  If those access are predictable, you
  15are better to use the macros provided by include/asm/unaligned.h.  The
  16alignment trap can fixup misaligned access for the exception cases, but at
  17a high performance cost.  It better be rare.
  19Now for user space applications, it is possible to configure the alignment
  20trap to SIGBUS any code performing unaligned access (good for debugging bad
  21code), or even fixup the access by software like for kernel code.  The later
  22mode isn't recommended for performance reasons (just think about the
  23floating point emulation that works about the same way).  Fix your code
  26Please note that randomly changing the behaviour without good thought is
  27real bad - it changes the behaviour of all unaligned instructions in user
  28space, and might cause programs to fail unexpectedly.
  30To change the alignment trap behavior, simply echo a number into
  31/proc/cpu/alignment.  The number is made up from various bits:
  33===             ========================================================
  34bit             behavior when set
  35===             ========================================================
  360               A user process performing an unaligned memory access
  37                will cause the kernel to print a message indicating
  38                process name, pid, pc, instruction, address, and the
  39                fault code.
  411               The kernel will attempt to fix up the user process
  42                performing the unaligned access.  This is of course
  43                slow (think about the floating point emulator) and
  44                not recommended for production use.
  462               The kernel will send a SIGBUS signal to the user process
  47                performing the unaligned access.
  48===             ========================================================
  50Note that not all combinations are supported - only values 0 through 5.
  51(6 and 7 don't make sense).
  53For example, the following will turn on the warnings, but without
  54fixing up or sending SIGBUS signals::
  56        echo 1 > /proc/cpu/alignment
  58You can also read the content of the same file to get statistical
  59information on unaligned access occurrences plus the current mode of
  60operation for user space code.
  63Nicolas Pitre, Mar 13, 2001.  Modified Russell King, Nov 30, 2001.