Re: [ECOS] Acess unaligned address

[Jonathan Larmour <jlarmour at redhat dot com>]

james chen wrote:
> 
> Hello, When I debug example on my test board ( based on ARM7TDI), run into
> __unpack_f function. it will stop at the instruction "ldr r3, [r0,
> 2]"( r0's value is word aligned ) and report bus error. I think that CPU has
> access unaligned adress(see below document, get it from web page). so what
> can I do to avoid it.

>From the gcc manual:

`-malignment-traps'
     Generate code that will not trap if the MMU has alignment traps
     enabled.  On ARM architectures prior to ARMv4, there were no
     instructions to access half-word objects stored in memory.
     However, when reading from memory a feature of the ARM
     architecture allows a word load to be used, even if the address is
     unaligned, and the processor core will rotate the data as it is
     being loaded.  This option tells the compiler that such misaligned
     accesses will cause a MMU trap and that it should instead
     synthesise the access as a series of byte accesses.  The compiler
     can still use word accesses to load half-word data if it knows
     that the address is aligned to a word boundary.

     This option is ignored when compiling for ARM architecture 4 or
     later, since these processors have instructions to directly access
     half-word objects in memory.

IIRC what we do on all our ARM ports is disable MMU alignment traps. If you
want to enable them, you will need to rebuild gcc, and edit
gcc/config/t-arm-elf to add either a multilib for -malignment-traps, or if
you don't mind making the tools completely this way, just add
-malignment-traps to TARGET_LIBGCC2_CFLAGS

One thing I'm slightly confused about is that libgcc _should_ have
defaulted to bulding for the arm7tdmi, which is a V4 architecture I
believe.

Jifl
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