Calculating the size of strings and program size?

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wht36

Joined: 18 Sep 2005
Posts: 106

wht36 08 Nov 2008, 15:01

Hello everyone, sorry for asking, I am sure these questions have been answered before, I just can't quite find the right thread with my searches...

So how do I calculate the size of strings and program size?

Bascially I am trying to write a byte coder for nwscript (http://web.archive.org/web/20041021071359/www.torlack.com/index.html?topics=nwndata_ncs) using FASM's macroinstructions. However this is the first time I am trying to write my own macros...

For example

Code:

; macro to code bytes
macro CONSTS op1, op2, [string] ;Place Constant String Onto the Stack
{
    db 4,5 ; byte code for CONSTS in nwscript
    dd .... ; <---- size of string goes here
    db string
}

; header
db       "NCS V1.0B"
dd     .... ; <---- size of program goes here

I think I need to use $ - . or sizeof.xxx , but I am not quite sure how to use them... Please help!

08 Nov 2008, 15:01

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 08 Nov 2008, 15:26

for string size do something like

Code:

string db 'Hello, World!',0
stringsize dd $-string-1

The minus one makes up for the extra zero at end.

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08 Nov 2008, 15:26

wht36

Joined: 18 Sep 2005
Posts: 106

wht36 08 Nov 2008, 15:37

Thanks! But the size of string needs to be assigned before the actual string is defined though (kind of like PASCAL style string).

08 Nov 2008, 15:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20454
Location: In your JS exploiting you and your system

revolution 08 Nov 2008, 15:42

maybe this:

Code:

; macro to code bytes
macro CONSTS op1, op2, [string] ;Place Constant String Onto the Stack
{
    local stringsize, stringstart
    db 4,5 ; byte code for CONSTS in nwscript
    dd stringsize ; <---- size of string goes here
    stringstart:
    db string
    stringsize=$-stringstart
}

; header
db      "NCS V1.0B"
dd      .... ; <---- size of program goes here

08 Nov 2008, 15:42

wht36

Joined: 18 Sep 2005
Posts: 106

wht36 08 Nov 2008, 16:08

Thanks very much, I applied a similar thing to get the size of program, and tested out just now, worked very nicely (assembled and decompiled to same output). Thanks again!

One thing though, is it possible to make a macro for the header so that I can make the file into an include file? (instead of putting a label for the start and a constant to calculate the size at the end of each program)

For example

Code:

macro header {
   db      "NCS V1.0B"
   dd      .... ; <---- size of program goes here 
}

08 Nov 2008, 16:08

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 08 Nov 2008, 16:17

wht36,

In short, no. How assembler supposed to know program size before it translates the source?

08 Nov 2008, 16:17

wht36

Joined: 18 Sep 2005
Posts: 106

wht36 08 Nov 2008, 16:48

Sigh... I suppose not. I will probably have to make two macros, one to put at the start and the other at the end....

08 Nov 2008, 16:48

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 08 Nov 2008, 17:05

wht36,

Beware of org, you may wish to redefine it too.

08 Nov 2008, 17:05

wht36

Joined: 18 Sep 2005
Posts: 106

wht36 08 Nov 2008, 22:38

Thanks for your advice! I will keep it in mind and hope I don't need to redefine it.

For those who also play the game Neverwinter Nights, I've uploaded the bytecoder to http://nwvault.ign.com/View.php?view=Other.Detail&id=1357

08 Nov 2008, 22:38

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