support of unaligned data access

[loeffel]

Posted on January 09, 2009 at 03:40

support of unaligned data access

[loeffel]

Posted on May 17, 2011 at 12:31

Hi

Are there any C compiler which supports the unaligned data access of the Cortex-M3 core? Right now I am using the Keil RVMDK. It seems that I have to use the __packed keyword for each variable, structure... to get unaligned data access but how do I get unaligned data access for a entire project? Are there better solutions than using the __packed keyword?

[joseph239955]

Posted on May 17, 2011 at 12:31

Hi there,

Why do you want unaligned data to be supported on all variables?

If the data elements in data structures are arranged carefully, you won't be wasting memory space anyway.

Joseph

[sjo]

Posted on May 17, 2011 at 12:31

From the armv7 docs:

The following data accesses always generate an alignment fault:

• Non halfword-aligned LDREXH and STREXH

• Non word-aligned LDREX and STREX

• Non word-aligned LDRD, LDM{IA}, LDMDB, POP, and LDC

• Non word-aligned STRD, STM{IA}, STMDB, PUSH, and STC.

The following data accesses support unaligned addressing, and only generate alignment faults when the

CCR.UNALIGN_TRP bit is set (see The System Control Block (SCB) on page B2-7):

• Non halfword-aligned LDRH, LDRHT, LDRSH, LDRSHT, STRH and STRHT

• Non halfword-aligned TBH

• Non word-aligned LDR{T} and STR{T}.

Cheers

Spen

[picguy]

Posted on May 17, 2011 at 12:31

FWIW, LDM/STM do not work unless word aligned. I suspose that its someplace in some doc file. 'Twaz a pain to rewrite to assure aligned load/store multiple.

[loeffel]

Posted on May 17, 2011 at 12:31

Hi

Thanks for the feedbacks. Additional information about this topic could be found at the cross-post on the Keil forum.

http://www.keil.com/forum/docs/thread13940.asp#msg69667

Regards