Consider the following code (help please)
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  1. #1
    Join Date
    May 2013
    Posts
    1

    Question Consider the following code (help please)

    Im working on some homework and have gotten stuck.

    Here is the problem I am working on.

    Consider the following source code, where R, S, and T are constants declared with
    Code:
    #define:
    int A[R][S][T];
    
    int store_ele(int i, int j, int k, int *dest)
    {
    *dest = A[i][j][k];
    return sizeof(A);
    }
    In compiling this program, gcc generates the following assembly code:

    i at %ebp+8, j at %ebp+12, k at %ebp+16, dest at %ebp+20

    1 movl 12(%ebp), %edx
    2 leal (%edx,%edx,4), %eax
    3 leal (%edx,%eax,2), %eax
    4 imull $99, 8(%ebp), %edx
    5 addl %edx, %eax
    6 addl 16(%ebp), %eax
    7 movl A(,%eax,4), %edx
    8 movl 20(%ebp), %eax
    9 movl %edx, (%eax)
    10 movl $1980, %eax

    A. Extend Equation 3.1 from two dimensions to three to provide a formula for
    the location of array element A[i][j][k].

    B. Use your reverse engineering skills to determine the values of R, S, and T
    based on the assembly code.

    I kind of know how to read this, I know that they are all being stored as a group under A, and j gets moved to the edx register in line 1.

    However when it comes to the actual questions I dont know what it means by Extend equation 3.1 or how to come up with a formula for the location of A[i][j][k]. To me it appears that the final value is moved to the eax register in the last line.

    Moving on to B, I do not know how to tell which values are corresponding to R, S, and T because you are not give initial locations of these variables.

    Any answers, suggestions, or assistance would be greatly appreciated, thank you very much for your time and help.

  2. #2
    Join Date
    Aug 2009
    Location
    Finally back in Alaska
    Posts
    141

    Re: Consider the following code (help please)

    However when it comes to the actual questions I dont know what it means by Extend equation 3.1 or how to come up with a formula for the location of A[i][j][k]. To me it appears that the final value is moved to the eax register in the last line.
    return values are generally returned in eax. though I think the *dest is being used as a return value also.

    To extend the code is relatively easy, youre just turning a two dimensional array into a three dimensional array. Im not sure how most hlls implement arrays, but at the assembly level, it is a single dimensional array, it just hides the math behind it. To access a specific element in the array youd follow a particular algorithm. in the above example:

    Code:
    *dest = A[i][j][k];
    the math would look something like *dest=A[(i*SizeOfDimensionJ*SizeOfDimensionK)+(j*SizeOfDimensionK)+k]; (I think thats it at least).

    I think R==i
    S==j
    and T==k

    I believe line 1 moves j into edx, lines 2-6 are computing the index into the array and storing it in eax, line 7 retrieves the element at that position, line 8 saves the value in *dest, Im not sure what line 9 is doing, then line 10 is storing 1980 in eax for the "return sizeof(A);" instruction.

    if I am reading this right: http://en.wikibooks.org/wiki/X86_***...operand_syntax, then I think the formula would translate into something like:

    line 2:eax=edx+edx*4==edx*5
    line 3:eax==edx+eax*2==edx+(edx*5)*2==edx+edx*10==edx*11 (so here you have j*11);
    line 4:edx==99*i
    line 5:eax=eax+edx==j*11+i*99
    line 6:eax=eax+k==j*11+i*99+k

    Be warned, I could be full of crap though since Ive never used gcc before.
    Last edited by AKRichard; May 5th, 2013 at 04:54 AM.

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