CODEWITHSUNDEEP
cassemblyx86gdbatt
Jaun7707
Jaun7707

Reputation: 119

What does cmp %eax,0x80498d4(,%ebx,4) mean?

I know there are some other questions similar to this, but I'm still having trouble understanding the () part of it. Could someone spell this syntax out for me? Thanks.

Upvotes: 0

Views: 1905

Answers (1)

starpax
starpax

Reputation: 130

cmp %eax,0x80498d4(,%ebx,4)

cmp is the comparison assembly instruction. It performs a comparison between two arguments by signed subtracting the right argument from the left and sets a CPU EFLAGS register. This EFLAGS register can then be used to do conditional branching / moving, etc.

First argument: `%eax (the value in the %eax register)

Second argument: 0x80498d4(,%ebx,4). This is read as offset ( base, index, scale ) In your example, the value of the second argument is the memory location offset 0x80498d4 + base (which I believe defaults to zero if not included) + value in %ebx register * 4 (scaling factor).

Note: I believe base here is empty and defaults to the value 0.

You can take a look at http://docs.oracle.com/cd/E19120-01/open.solaris/817-5477/ennby/index.html for more information on the syntax for Intel x86 assembly instructions.

Upvotes: 2

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