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Using REV16 in C Code

Robert S

Hi guys,

First of all, I'm using a LM3S6950 Arm Cortex M3 microcontroller with Keil MDK 3.70. I'm programming in C, Thumb Mode.

For converting low endianess to big endianess I want to use the rev and rev16 instruction.

32 Bit swaps can be done with the build in __rev() inline C function (http://www.keil.com/support/man/docs/armccref/armccref_cjaeegce.htm)

How can I do this with the 16 bit instruction since inline assembler isn't supported in thumb mode?

Thanks for your replies
br Robert

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  • 0 in reply to Ashley Madison

    Parentheses around "a" is only needed if you make a #define of the expression.

    Remember that the ARM is a 32-bit processor. Even if variable a is 16-bit large, the compiler will upgrade your expression to a 32-bit values, so if b is 32-bit and you try to reverse 0x8765, you will not get 0x6587 but 0x876587 if you don't do the bit-and.

    And if a happens to be signed, life will be extra fun. With the & operations, you would still get the correct result, but without them, you would get 0xffffff87 because the upgrade from 16-bit to 32-bit will be with sign-extend.

  • 0 in reply to Ashley Madison

    ...just in case (a) is an expression.

    also, I am not sure why you need to to the & operation before shifting, assuming a is 16bit or shorter.

    I realize that. What I did was copy&paste from this presentation (page 4):
    www.newark.com/.../LMTTCM3.pdf