Assembly Code

[limpid lance]

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[valid mist]

!f

[limpid lance]

So i have questions about this problem:

The following code transposes the elements of an M × M array, where M is a constant defined by #define:

void transpose(long A[M][M]) {
  long i, j;
  for (i = 0; i < M; i++)
    for (j = 0; j < i; j++) {
      long t = A[i][j];
      A[i][j] = A[j][i];
      A[j][i] = t;
    }
}

and im given the following assembly code for the inner loop:

.L6 : movq(% rdx), % rcx movq(% rax), % rsi movq % rsi, (% rdx) movq % rcx,
    (% rax) addq $8, % rdx addq $120, % rax cmpq % rdi, % rax jne.L6

A. Which register holds a pointer to array element A[i][j]?

B. Which register holds a pointer to array element A[j][i]?

C. What is the value of M
I know the answers but I dont understand them.

shengzhu

Formatted by Monke

[valid mist]

I know the answers but I don't understand them
Provide these answers and why you don't understand them.

Also probably better suited for #asm-arch-osdev

[wide linden]

when you look at the first 4 instructions, you can see that it's a standard swap:

(rdx) -> rcx -> (rax)
and at the same time:
(rax) -> rsi -> (rdx)

rdx and rax hold pointers to things, and the contents those pointers are pointing to are swapped. looking at the latter instructions, you can see that one pointer rax is incremented by 8, while the other pointer rdx is incremented by 120. since rax is incremented by a smaller amount, it must be the term where the least significant index is incrememted A[i][j] and rdx must be the term where the most significant index is incremented A[j][i]

the distance between A[i][j] and A[i][j+1] is sizeof(long) whereas the distance between A[j][i] and A[j+1][i] is M*sizeof(long)

[limpid lance]

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