问题
ar db "Defference $"
What's the difference between
mov dx,offset ar
and
lea dx,ar
I think both are doing same work, but what is the difference between these two
回答1:
In this use-case LEA and MOV do the same thing. LEA is more powerful than MOV if you want to calculate an address in a more complex way.
Lets for example say you want to get the address of the n'th character in your array, and the n is stored in bx. With MOV you have to write the following two instructions:
Mov dx, offset ar
add dx, bx
With lea you can do it with just one instruction:
lea dx, [ar + bx]
Another thing to consider here: the add dx,bx instruction will change the status flags of the CPU. The addition done inside the lea dx, [ar + bx] instruction on the other hand does not change the flags in any way because it is not considered an arithmetic instruction.
This is sometimes helpful if you want to preserve the flags while doing some simple calculations (address calculations are very common). Storing and restoring the flag-register is doable but a slow operation.
回答2:
Quote from Assembly Language for x86 Processors, 7e, KIP R. IRVINE
It is not possible to use OFFSET to get the address of a stack parameter because OFFSET only works with addresses known at compile time. The following statement would not assemble:
mov esi,OFFSET [ebp-30] ; error
来源：https://stackoverflow.com/questions/2797432/difference-between-lea-and-offset