Addl instruction explanation

Question

I'm studying assembly language and can't resolve the following exercise myself.

Assume the following values are stored at the indicated memory addresses and registers:

Now, we have an instruction:

addl %ecx , (%eax)

For me it means - storing the result of addition of values stored in %ecx and in memory address (%eax), in a memory address (%eax).

Correct answer for that exercise is : Value 0x100 and destination address 0x100.

I understand that right operand is destination address, but how did we get value of 0x100 by the calculation %ecx + (%eax)?

Solution

First, I hate AT&T syntax, which is what you have here... that aside.

EAX contains 0x100. 0x100 has the value 0xFF in it.

ECX contains 0x1.

0x1 + 0xFF = 0x100. So far so good.

The final result is then placed into the address pointed to by EAX. Therefore, (0X100) == 0x100

I think you were most of the way there.

Comments

[xotix]

Thanks for the explanation. I want to add: It also helps to think of addX src, dest as dest += src so in C: dest = dest + src. Also note that parentheses notate a pointer. The parentheses basically tell us "treat the value as an address and follow it", so basically "dereferencing" it. But attention: When using e.g. the LEA instruction, the parentheses don't dereference the pinter since LEA doesn't really touch memory. That's correct?

[David Hoelzer]

You have that right, @xotix. Check this out: stackoverflow.com/questions/1658294/…