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msp430-elf-gcc intrinsic __swap_bytes() can only handle word aligned data

friedl
Intellectual 900 points

I've used the intrinsic __swap_bytes(x) to do big to little endian data conversion.

I came to the conclusion that __swap_bytes(x) only works correctly when x is word aligned.

This intrinsic is essential the assembler command "swpb x", the generated code is shown below:

    9898:	2e 4d       	mov	@r13,	r14	;
    989a:	8e 10       	swpb	r14		;
    989c:	8d 4e 00 00 	mov	r14,	0(r13)	;

The problem stems from the fact that 'mov @r13, r14' seems to only be able to do word transfers, at least that's what happens when this command is executed by the CPU.

GCC (msp430-14r1-167) does not seem to be able to take this into account and work around this alignment problem.

, could you possibly comment on that?

  • 0
    TI__Guru**** 245120 points

    friedl said:
    I came to the conclusion that __swap_bytes(x) only works correctly when x is word aligned.

    Yes.  The __swap_bytes built-in expects and returns an unsigned short.  The unsigned short type requires an alignment of 2-bytes (16-bits).  I cannot think of a way to pass an unaligned address to __swap_bytes without violating the C standard.  The TI compiler handles all of this the same way.

    Thanks and regards,

    -George