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Alignment of variables for Cortex-M0

Alistair Lowe
Note: This was originally posted on 12th October 2012 at http://forums.arm.com

[font=verdana, sans-serif][size=2]Hi guys,[/size][/font]

[font=verdana, sans-serif][size=2]I'm currently working with a Cortex-M0 (using armcc), which is incapable of unaligned reads.[/size][/font]

[font=verdana, sans-serif][size=2]I have a byte array which is storing a received packet in a certain protocol, the first lot of information to come in is 1 byte, followed by two 16-bit half-words.[/size][/font]

[font=verdana, sans-serif][size=2]This is leading to misalignment and preventing me from performing any direct casting/pointer access to these values; I can't space the data out as it's being placed in the array by DMA.[/size][/font]

[font=verdana, sans-serif][size=2]I know with a structure it can be packed, however I've had no such luck with pointers and access to my array.[/size][/font]

[font=verdana, sans-serif][size=2]Is it possible to ask the compiler to align the start of my byte array on the edge of a word (3 bytes), so that the rest of my data is aligned? And if so how?[/size][/font]

[font=verdana, sans-serif][size=2]Any other suggestions?[/size][/font]

[font=verdana, sans-serif][size=2]Many thanks for any help and advice.[/size][/font]

Parents
  • Note: This was originally posted on 14th October 2012 at http://forums.arm.com


    You can mark a pointer as being packed:

    __packed short* pUnalignedHalfWord = (__packed short*) xyz;

    So if you had a structure:


    __packed struct MyStruct_t
    {
      char a;
      short b;
    };

    MyStruct_t foo;

    __packed short* pUnalignedHalfWord = (__packed short*) &(foo.B);




    Thank you for the feedback. I had actually tried this though with no luck, I will explore further and see if I can make this work, else return with some sample code/errors for some useful debugging!
Reply
  • Note: This was originally posted on 14th October 2012 at http://forums.arm.com


    You can mark a pointer as being packed:

    __packed short* pUnalignedHalfWord = (__packed short*) xyz;

    So if you had a structure:


    __packed struct MyStruct_t
    {
      char a;
      short b;
    };

    MyStruct_t foo;

    __packed short* pUnalignedHalfWord = (__packed short*) &(foo.B);




    Thank you for the feedback. I had actually tried this though with no luck, I will explore further and see if I can make this work, else return with some sample code/errors for some useful debugging!
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