What does MOV EAX,DWORD PTR DS:[ESI+EBP*8] do?

Question

If I do step through the debugger in Ollydbg I see

MOV EAX,DWORD PTR DS:[ESI+EBP*8]

and register ESI = 0040855C and EBP = 00000000.

My problem is I dont know 2 register * 8

Answer 1

In normal INTEL syntax this instruction moves a value from memory into EAX.

MOV EAX,DWORD PTR DS:[ESI+EBP*8]

It is usually used to extract a value from an array.
The array is situated in memory at DS:ESI.
The elements are indexed through EBP.
The scale of 8 means that every element is 64 bit long and this instruction only reads the low dword.

Answer 2

MOV EAX,DWORD PTR DS:[ESI+EBP*8]

MOV - move

EAX - to EAX (generally this will be a value you just calculated)

DWORD PTR - from the value pointed at by

[DS: - in the data segment]

[ESI+EBP*8] - ESI plus 8 times EBP.

Move the value in EAX into the address pointed at by ESI + EBP*8 (ESI plus 8 times EBP, it means exactly how it's written)

This is probably being used to load data from an array, where the 8 is there to scale up the counter (which is EBP) to the size of the thing being stored (8 bytes), and ESI contains the address of the start of the array. So if EBP is zero, you store the data in ESI+0, if EBP=1, you end up storing at ESI+8, etc.