Given the following code:
L1 db "word", 0
mov al, [L1]
mov eax, L1
What do the brackets ([L1]) represent?
Given the following code:
L1 db "word", 0
mov al, [L1]
mov eax, L1
What do the brackets ([L1]) represent?
[L1]
means the memory contents at address L1. After running mov al, [L1]
here, The al
register will receive the byte at address L1 (the letter 'w').
Operands of this type, such as [ebp]
, are called memory operands.
All the answers here are good, but I see that none tells about the caveat in following this as a rigid rule - if brackets, then dereference, except when it's the lea
instruction.
lea
is an exception to the above rule. Say we've
mov eax, [ebp - 4]
The value of ebp
is subtracted by 4 and the brackets indicate that the resulting value is taken as an address and the value residing at that address is stored in eax
. However, in lea
's case, the brackets wouldn't mean that:
lea eax, [ebp - 4]
The value of ebp
is subtracted by 4 and the resulting value is stored in eax
. This instruction would just calculate the address and store the calculated value in the destination register. See What is the difference between MOV and LEA? for further details.
Simply means to get the memory at the address marked by the label L1.
If you like C, then think of it like this: [L1]
is the same as *L1
The brackets mean to de-reference an address. For example
mov eax, [1234]
means, mov the contents of address 1234 to EAX. So:
1234 00001
EAX will contain 00001.
Direct memory addressing - al
will be loaded with the value located at memory address L1
.
As with many assembler languages, this means indirection. In other words, the first mov
loads al
with the contents of L1
(the byte 'w'
in other words), not the address.
Your second mov
actually loads eax
with the address L1
and you can later dereference that to get or set its content.
In both those cases, L1
is conceptually considered to be the address.
They mean that instead of moving the value of the register or numeric value L1
into the register al
, treat the register value or numeric value L1
as a pointer into memory, fetch the contents of that memory address, and move that contents into al
.
In this instance, L1 is a memory location, but the same logic would apply if a register name was in the brackets:
mov al, [ebx]
Also known as a load.
It indicates that the register should be used as a pointer for the actual location, instead of acting upon the register itself.