Re: [ECOS] Acess unaligned address

["james chen" <james_ch1 at sina dot com>]

Hello, Jifl:
   I am sorry that I make a mistake. I use the wrong CPU(arm7di) instead of
arm7tdmi in my make file, so it cause error.Now it works well.
thank you very much!
Best Regards,
james

> 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
> --
> Red Hat, Rustat House, Clifton Road, Cambridge, UK. Tel: +44 (1223) 271062
> Maybe this world is another planet's Hell -Aldous Huxley || Opinions==mine
>
>