问题
I'm very new to assembly, and have some very basic questions.
What is the difference between these four commands?
mov ebx, eax
mov [ebx], eax
mov ebx, [eax]
mov [ebx], [eax]
They say that the brackets mean "get the value of the address". But what, then, does that very first line really do? Does it not move the value of eax into ebx? If it does, then what are the point of the brackets?
回答1:
Let's make a very simple example and imagine we have a CPU with only two registers, EAX and EBX.
mov ebx, eax
Simply copies the value in eax to the ebx register
| EAX : 0123456 | ----> | EAX : 0123456 |
| EBX : 0000000 | ====> | EBX : 0123456 |
Now let's add some memory space
ADDRESS VALUE
000000 6543210
000004 5189784
000008 1698791
00000C 9816517
000010 9816875
000014 5498156
mov [ebx], eax
Moves the value in eax to the memory address contained in ebx.
| EAX : 0123456 | --no--> | EAX : 0123456 |
| EBX : 0000008 | --change--> | EBX : 0000008 |
ADDRESS VALUE VALUE
000000 6543210 ----> 6543210
000004 5189784 ----> 5189784
000008 1698791 ====> 0123456
00000C 9816517 ----> 9816517
000010 9816875 ----> 9816875
000014 5498156 ----> 5498156
mov ebx, [eax]
Moves the value from the memory address contained in eax to ebx.
| EAX : 0000008 | ----> | EAX : 0000008 |
| EBX : 0123456 | ====> | EBX : 1698791 |
ADDRESS VALUE
000000 6543210
000004 5189784
000008 1698791
00000C 9816517
000010 9816875
000014 5498156
mov [ebx], [eax]
This, finally, you would think would move the value from the memory address contained in eax to the memory address contained in ebx.
| EAX : 0000008 | --no--> | EAX : 0000008 |
| EBX : 000000c | --change--> | EBX : 000000c |
ADDRESS VALUE VALUE
000000 6543210 ----> 6543210
000004 5189784 ----> 5189784
000008 1698791 ----> 1698791
00000C 9816517 ====> 1698791
000010 9816875 ----> 9816875
000014 5498156 ----> 5498156
But this combination is disallowed by the x86 architecture. You cannot move from memory to memory.
The use of brackets is therefore equivalent to a dereferencing operation.
回答2:
You were missing the operand delimiter , in the instructions. I don't know (yet) of any assembler without it. I fixed that in the quotes.
In x86 assembly some registers can be used as data registers or as address registers (a difference to other architectures). These registers are called GPRs ("General Purpose Registers"). They can contain 32-bit-values or 32-bit addresses. Their "names" are EAX,EBX,ECX,EDX,ESI,EDI,ESP,EBP.
mov ebx, eax
does move the value in EAX to EBX.
mov [ebx], eax
does move the value in EAX to the 32-bit DWORD value pointed to by the 32-bit address in EBX
mov ebx, [eax]
does move the 32-bit DWORD value pointed to by the 32-bit address in EAX to EBX
mov [ebx], [eax]
is an invalid instruction in 32-bit Intel assembly, because basic x86 assembly does not support two memory operands in one (two-operand) instruction. Newer instructions (SSE, AVX) with three or four operands are able to use more than one memory operand. This is a result of a more complex instruction encoding (using instruction prefixes).
来源：https://stackoverflow.com/questions/48608423/what-do-square-brackets-mean-in-x86-assembly