CODEWITHSUNDEEP
cassemblyx86gdb
user994165
user994165

Reputation: 9512

Assembly Jump Instruction Address Calculation

I see the following instruction in gdb

jmp    *0x804a09c(,%eax,4)

Just prior to exececution I types these commands:

(gdb)p/x *0x804a09c
$40 = 0x8048e0e

(gdb) p $eax
$41 = 6

So when I try to calculate the address I will jump to, I get:

(gdb) p/x *0x804a09c + 4*$eax
0x8048e26

However, the jump actually goes to address 0x8048ead. What's wrong with my calculation?

Upvotes: 0

Views: 1083

Answers (1)

user1129665
user1129665

Reputation: 

jmp    *0x804a09c(,%eax,4)

means to jump to the address that is stored at the result of this calculation0x804a09c(,%eax,4), the * on the whole not on just 0x804a09c. AT&T syntax can be misleading, Intel syntax is more clear here:

jmp    DWORD PTR [eax*4+0x804a09c]

so it should be:

(gdb) p/x *(0x804a09c + 4 * $eax)

Upvotes: 5

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