CODEWITHSUNDEEP
assemblyx86att
Unhandled Exception
Unhandled Exception

Reputation: 1513

MOV instruction with an offset

Here is a line of assembly code from a core file. 

0x00002ac8c957012d <+45>:    mov    0x8(%rsp), %rsi

I noticed that registry rsp is at the following location so I use the Examine command to view the content plus 16 more objects/bytes.

> x/16x 0x2acd5752a610
0x2acd5752a610: 0x70   0xa6   0x11   0xff   0x29   0xcd   0x22   0x00
0x2acd5752a618: 0x33   0xee   0xf1   0xa9   0xb2   0xcc   0x34   0x76

When figuring out the offset of 0x8 is this going to be 0 based resulting in the value being 0x33 or just count starting with 1 to 8 from the start which results in the value being 0x00?

Upvotes: 0

Views: 5045

Answers (1)

Johan
Johan

Reputation: 76617

You are confused by the braindead abomination that is AT&T syntax.

In Intel's intended syntax mov 0x8(%rsp), %rsi translates to:

mov rsi,[rsp+8]

Which means fill the 8 bytes of register rsi with the 8 bytes at address [rsp+8].
Note that rsp is a 64-bit (8 byte) register which handles 8 bytes at a time.
We don't know what the value of rsp is, but we do know that rsp is the stack pointer and +8 means we take the address 8 bytes above the stack pointer, or the second most recently pushed value on the stack (because the stack grows downward).

This instruction is sneaking a peak inside the stack :-)

Upvotes: 4

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