question about instruction times 512-($-$$) db 0

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adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 21 Apr 2004, 20:24

I'm slowly moving to FASM from NASM. NASM had a 'times' directive that made the file a certian size by filling the rest with zero-bits. This is extremely useful in bootsectors (they MUST be 512 bytes). For example

...
times 512-($-$$) db 0
...

would make a 512 byte file.
Does FASM support such a directive?
Prompt response would be appreciated

-Anthony DeFeo

21 Apr 2004, 20:24

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8390
Location: Kraków, Poland

Tomasz Grysztar 21 Apr 2004, 20:37

Code:

times 512-$ db 0

will work with fasm.

21 Apr 2004, 20:37

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 23 Apr 2004, 16:32

Privalov wrote:

Code:
times 512-$ db 0    
will work with fasm.

I tried this and it does not work. It returns an error about an invalid something (i forget ) I am using the latest Linux version of FASM on a Debian Linux 2.1 system with kernel 2.2.12

Why is it not working

Question

23 Apr 2004, 16:32

roticv

Joined: 19 Jun 2003
Posts: 374
Location: Singapore

roticv 23 Apr 2004, 16:38

Weird the following work for me (part of my hack and slash bootloader)

Code:

times 512-$-2 db 0
db 55h,0aah ; boot signature

23 Apr 2004, 16:38

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8390
Location: Kraków, Poland

Tomasz Grysztar 23 Apr 2004, 17:36

First, this can be used mainly with binary output format only - when you use some relocatable format (like object files, but also PE), you cannot use $ symbol this way, because it doesn't have fixed value.

Second, expression 512-$ will be of course negative when $ is larger than 512 and you will get error in such situation, too.

23 Apr 2004, 17:36

asmdemon

Joined: 18 Jan 2004
Posts: 97
Location: Virginia Beach, VA

asmdemon 23 Apr 2004, 23:14

this is what i always use and it works all the time

Code:

times $-510 db 0
db 0x55h, 0xaa ; boot signature

_________________
It is better to be on the right side of the devil than in his path.

23 Apr 2004, 23:14

Daemon

Joined: 08 Jul 2003
Posts: 15

Daemon 24 Apr 2004, 05:59

Why not to use a 'rb' directive ?

btw question to Privalov: is this directive always leaves zeros or not?
I mean is it possible that in

rb 1024

can be not only zeros but some random data ??

24 Apr 2004, 05:59

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 24 Apr 2004, 07:01

manual says you should always treat value as --undefined--, and that is. If "rb" is used at end of file, it will just reserve space in file header, and thus when you run program memory will contain data remained from previous owner of this part of memory

24 Apr 2004, 07:01

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 26 Apr 2004, 19:48

Thanks! I'll try it when I get home from school.

26 Apr 2004, 19:48

adefeo

Joined: 12 Jan 2004
Posts: 46
Location: Bellmore, Long Island, New York

adefeo 27 Apr 2004, 19:22

NEITHER of the methods work... I'm Confuzzled...What's wrong:?:

27 Apr 2004, 19:22

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