CODEWITHSUNDEEP
assemblyx86
grivescorbett
grivescorbett

Reputation: 1625

x86 memory addressing with function parameters

So if I have a procedure where the first formal parameter is an int[] and I'm enumerating through that loop, I'm confused about why one piece of code works where another doesn't. I should be able to do this:

#where ebp+8 is the location of the pointer, and ecx is the counter
mov edx, [ebp+ecx*4+8]

This gives me a gibberish value for edx, but this code works fine

mov edx, [ebp+8]
mov edx, [edx+ecx*4]

I don't understand the difference between those statements.

Upvotes: 4

Views: 360

Answers (1)

Mysticial
Mysticial

Reputation: 471229

They are different:

In the first code:

mov edx, [ebp+ecx*4+8]

You are loading from the address: ebp+ecx*4+8

In the second code:

mov edx, [ebp+8]
mov edx, [edx+ecx*4]

You first load the value stored at ebp+8. Then you use it as the base address for the second load.

In other words, the base address is stored at the memory location pointed to by ebp + 8. It is not actually stored in the ebp register itself.

Upvotes: 4

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