Print Page - I don't see difference between mov eax, ebp and lea eax, [ebp]

Title: I don't see difference between mov eax, ebp and lea eax, [ebp]
Post by: jackheroes on August 16, 2011, 10:12:20 AM

I don't see difference between "mov eax, ebp" and " lea eax, [ebp] "
I think lea instruction is troublesome here, it dereferences ebp and gets its' address again, like c++( p1 = &(*p) ).
Why don't just use mov eax, ebp. I have tested both statements, they resulted the same thing.

Title: Re: I don't see difference between mov eax, ebp and lea eax, [ebp]
Post by: qWord on August 16, 2011, 10:56:27 AM

Quote from: jackheroes on August 16, 2011, 10:12:20 AM
I don't see difference between "mov eax, ebp" and " lea eax, [ebp] "

same operation encoded in two different instructions.
In your case LEA does exact the same as MOV. However, LEA has the capacity to do some additional arithmetic (SIB).

Code Select

lea eax,[ebp+4*ecx-4]

Title: Re: I don't see difference between mov eax, ebp and lea eax, [ebp]
Post by: dedndave on August 16, 2011, 02:07:32 PM

you just need to choose the right instruction for the job
in your case, i suspect MOV is the right one

LEA is used if the CPU needs to calculate the address
for example, if you wanted the address of [EBP+4], rather than [EBP]

Code Select

lea eax,[ebp+4]
that is like moving EBP into EAX, then adding 4 to it

Title: Re: I don't see difference between mov eax, ebp and lea eax, [ebp]
Post by: raleeper on August 17, 2011, 01:23:29 AM

The 2 instructions assemble as:
mov, eax, ebp 8bc5
lea, eax, [ebp] 8d4500

but

lea, eax, [ebp+4] 8d4504

Title: Re: I don't see difference between mov eax, ebp and lea eax, [ebp]
Post by: dedndave on August 17, 2011, 01:55:07 AM

yes - all the [EBP] intructions are coded with a byte offset

8D4500 is actually

Code Select

lea [ebp+00]

The MASM Forum Archive 2004 to 2012

General Forums => The Campus => Topic started by: jackheroes on August 16, 2011, 10:12:20 AM