trouble understanding assembly command “load effective address” [duplicate]

Answer 1

The LEA instruction computes a memory address using the same arithmetic that a MOV instruction uses. But unlike the MOV instruction, the LEA instruction just stores the computed address in its target register, instead of loading the contents of that address and storing it.

Consider your first LEA instruction:

lea -0x18(%ebp), %ebx

This instruction computes the sum of -0x18 and the value in the EBP register. It gets some result S. It stores S in the EBX register.

In the addend -0x18, the “-” is a negative sign and the “0x” means it's a hexadecimal constant. So the addend is negative 18₁₆, which is -24₁₀. So this LEA instruction simply subtracts 24 from the value in EBP and stores the result in EBX.

Contrast this with your MOV instruction:

mov -0x4(%ebx), %eax

This instruction computes the sum of -0x4 and the value in the EBX register. It gets some result S. Then it fetches the value of the word at address S in memory, getting some value M. It stores M in the EAX register.

Answer 2

The LEA instruction loads the address that will be evaluated with the particular addressing mode into the destination register. Consider these two cases:

lea -0x18(%ebp), %ebx
mov -0x18(%ebp), %ebx

The first instruction loads the address calculated as offset -0x18 relative the current value of ebp into ebx. The second instruction loads the memory content at this address into ebx.

The negative offset means that location is below the address in a register and positive offset means that the location is above the address in a register. It is common to represent a memory with the zero address at bottom and the addresses grow toward the top in the drawings:

0xFFFFFFFC   !_______!
    ....
+0x4(ebp) -> !_______! 
(ebp) ->     !_______! 
-0x4(ebp) -> !_______! 
    ....
0x00000000   !_______!