Thread: Gnu inline assembler question

  1. #1
    Registered User
    Join Date
    Jul 2015

    Gnu inline assembler question


    I am currently trying to convert code from a old compiler to a new one. Specifically GNU Gcc/G++.

    Is it possible to return anything from a inline assembler call?

    for example:

    #ifdef HEAP_DEBUG 
    			unsigned long int iTemp;
    			iTemp = asm(" move.l __HEAP,d0"); 
    			iCurrentHeapSize = iTemp - iHeapBasePointer;
    			if(iCurrentHeapSize > iMaxHeapSize) { iMaxHeapSize =
    			iCurrentHeapSize; } 
    I keep getting a parse error while compiling and I am not sure why.

    Is the line of code in red the correct way to return values?
    (My assumption, I am not the original coder.)

    Thanks for your time.

  2. #2
    Programming Wraith GReaper's Avatar
    Join Date
    Apr 2009
    Yes, of course, but you need a different syntax. For example,
    this piece of assembly:
    MOV EAX, myVar
    MOV myVar, EAX
    would become:
    asm("movl %0, %%eax \n\t"
        "mull %%eax \n\t"
        "movl %%eax, %0 \n\t"
        : "=m"(myVar)
        : : "%eax");
    It is a little bit difficult, here is what you need to know about gcc inline assembly:

    The assembly seems so different because gcc uses AT&T syntax by default.
    Devoted my life to programming...

  3. #3
    Registered User
    Join Date
    Jul 2015
    Hey GReaper,

    Thanks for your response.

    It never occured to me that I would not have the correct syntax.

    Thanks for your input.

  4. #4
    Informer -Adrian's Avatar
    Join Date
    Jan 2013
    Regarding the Assembly itself, you can use Intel syntax with GCC if you specify it. This might be preferable if your old code base is written that way. I think it looks neater anyway.

  5. #5
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    Join Date
    Jul 2015

    I am using a specific version of the GNU compiler.

    It is supposed to support the M68k chipset.

    I was wondering if you happened to know if i could choose an assembly option for that desired type.

    thanks for your time.

  6. #6
    Informer -Adrian's Avatar
    Join Date
    Jan 2013
    Ah sorry, I had blindly assumed x86. You'll have to check if your version of GNU Assembler can work with the -masm=dialect flag. I hope it can.

    Edit: I haven't thought this through. I reckon with M68k there are probably not multiple choices. I don't know anything about it.
    Last edited by -Adrian; 07-14-2015 at 02:31 PM.

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