CODEWITHSUNDEEP
assemblyx86
akari
akari

Reputation: 149

Can you help me to explain this code [ASM]

I have a piece of small code and I can't understand what line 3 does. Can anyone help me to explain it. Many thanks.

mov     ebx, pointer1   //Assign of pointer pointer1 value to ebx
movzx   eax, byte ptr[esi]   //Get value from pointer address esi and assign to eax
mov     dx, [ebx + eax * 2]  //??? Dont know what it does
mov     [edi], dx

As I guess, If [ebx] is a byte array, that line will take array[eax_value] and array[eax_value+1]?

Upvotes: 1

Views: 88

Answers (2)

Devolus
Devolus

Reputation: 22074

Pointer1 is a pointer to a short int (16 bits) array. esi is pointing to some byte value which is an index in the array.

So the pointer is loaded to ebx, then the index is loaded into eax and multiplied by 2 (because of the 16 bit data size) and added as an offset to the array pointer. The value from the array is copied to wherever edi is pointing to.

Upvotes: 3

It'sPete
It'sPete

Reputation: 5211

ebx holds a pointer value to pointer1. eax is used to offset into it. You are taking the nth 16bit value from pointer1 where n is determined by the value read into eax.

Upvotes: 4

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