Lpc2000

On Tuesday 08 November 2005 14:40, Tom Walsh wrote:
> ee_gary wrote:
> >Hello,
> >
> >I'm having a problem where my code goes into the bushes after ~10
> >seconds.  Running it in the simulator results in a "Non-aligned
> >Access" error, ARM Instruction at 00000200H, Memory Access at
> >40000241H.  Any idea what that means?  Simple code, just an interrupt
> >driven timer and interrupt driven SPI…
>

What instruction was this accessing?

The following things can give you non-aligned accesses:

1) Trying to read a U32 on a non-4-byte boundary or a U16 on a non-2byte 
boundary. This might raise a data abort.

2) Trying to execute an ARM instruction at a non-4-byte location or a thumb 
instruction at a non-2-byte location. This might raise a prefetch abort.



> You can run into this when mixing Thumb with ARM code. When interrupt
> routines are entered, the processor is in ARM mode, either write your
> handlers in ARM or switch to THUMB mode, process the interrupt, then
> exit THUMB mode and return from interrupt.

Yes, this is possible since thumb addresses are marked by setting the least 
significant bit.  If you try executing the address by using a regular branch 
or bl then this will break. You need to use a bx.

>
> FWIW, Thumb, and thumb-internetworking, is too much trouble to deal
> with, I run strictly ARM mode.

To each their own, but I strongly disagree. I use thumb and interworking a 
lot. gcc support for this is great.

I always write all my assembly with interworking (ie using bx) regardless.

For an example you might look at the example stuff I posted on the AT91SAM7 
group.