[Asterisk-Dev] Asterisk Hardware Platform - Intel x86 versus Intel RISC Xscale (ARM)

[Holger Schurig]

> HS> One thing that is always an issue with a RISC type chip is
> HS> alignment. You might need to adjust your source code to get
> HS> alignment correct.
>
>    Can you clarify this?

When you have a memory structure like this:

+---------+
|  byte   |  00
+---------+
|         |  01
+         +
|         |  02
+  long   +
|         |  03
+         +
|         |  04
+---------+

then a x86 (or CISC) chip can access the 32 bit value with some penalty, 
but it can access it.

However, ARM chips (and Intel XScale is a sub-architecture of ARM) and 
some other RISC chips can't access that. They can only fetch data from 32 
bit boundaries. Therefore, the processor throws an exception.

The arm-version of the Linux kernel can catch this exception, use two 32 
bit accesses to get the 32 bit long. Then they assemble the data and 
return from the exception.

Unfortunately, this takes an awful lot of time.


Usually this happens only when you have external things force you to have 
this layout, e.g. it happened for me in fdisk (because of the layout of 
the MBR sector) or in libusb. Here, the format of some USB urb's had this 
weird alignment.

So, it's rare, but it happens. In your own programs, it usually doesn't 
happen, because you do either:

+---------+
|         |  00
+         +
|         |  01
+  long   +
|         |  02
+         +
|         |  03
+---------+
|  byte   |  04
+---------+

or

+---------+
|  byte   |  00
+---------+
| <empty> |  01
+---------+
| <empty> |  02
+---------+
| <empty> |  03
+---------+
|         |  04
+         +
|         |  05
+  long   +
|         |  06
+         +
|         |  07
+---------+

The latter is the default behavior of arm-linux-gcc.


There is some iaxclient based Phone software for ARM based PDAs available 
and they this alignment problem in the IAX protocol. Eventually a patch 
made it into iaxclient so that the assembly of the 32 bit long value was 
done in user-space, not in kernel, which is quite faster.

And because this was necessary in iaxclient, I think it will bite you in 
Asterisk as well.