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kamac



Joined: 06 Dec 2013
Posts: 12
kamac
Hey there.

I have lately seen something like that:

Code:
constString:
  dd 3
  dw 97,98,99
    


And I've been wondering how does it work since. I know that if I have something like that:

Code:
constString:
  dw 97,98,99    


Then it is a simple constant "string" of wchars. But what about the 1st example? Is this something like pseudo-struct? (dd 3 would allocate a 4-byte integer, and after that there would be the wchar array? If yes, how would I access separate elements?)

Also, I have seen something like that in BlitzBasic's ASM code:
dd _bbStringClass

Does anybody have a clue what could it possibly be? (the _bbStringClass)

Cheers.
Post 06 Dec 2013, 16:17
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shutdownall



Joined: 02 Apr 2010
Posts: 518
Location: Munich
shutdownall
Well constString is a label in your first example.
Could be followed by data of any type or even code (depending on segment rules).
Cool

The meaning of the first example is obviously a structure with a double word for length of string, followed by the string.
Post 06 Dec 2013, 16:55
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kamac



Joined: 06 Dec 2013
Posts: 12
kamac
Okay, so constString would point to the 1st element here, which is a 4-byte integer?

How can I access 2nd element (which is dw 97,98,99)?

I access 1st by doing:
mov eax,constString
Post 06 Dec 2013, 16:57
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shutdownall



Joined: 02 Apr 2010
Posts: 518
Location: Munich
shutdownall
No, this way you load the address of constString but not the first data.

Use:
Code:
mov ecx,[constString]
mov esi,constString+4
mov al,[esi]
    


This will initialize the counter register ecx with lenght of string and al with the first byte of the string. It is more comfortable to use lods (load string):

Code:
mov esi,constString
lodsd
mov ecx,eax
@@: lodsb
call printch
loop @b
    


Here you load esi with address of string.
lodsd loads a dword from [esi] in eax (the size)
lodsb loads a char from [esi] in al
loop repeats this as long as there are more characters.

Take notice that esi is automatically incremented after read depending on size. So lodsd will add 4 to esi and lodsb will increment by 1. There is a lodsw existing for reading words.

The same could be done with storing data using stos (store string), register edi instead of esi and instructions stosb, stosw, stosd in similar way.
Post 06 Dec 2013, 17:09
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kamac



Joined: 06 Dec 2013
Posts: 12
kamac
That's neat Smile

I'm quite green so I'll ask - do I have to use ecx in the 1st example, or is this only convenction?
Post 06 Dec 2013, 17:20
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nop



Joined: 01 Sep 2008
Posts: 165
Location: right here left there
nop
kamac wrote:
I'm quite green so I'll ask - do I have to use ecx in the 1st example, or is this only convenction?
yes cx is the counter register in all string instructions lods stos movs ect
Post 07 Dec 2013, 01:27
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