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flat assembler > Main > Is it possible to get the file size of a file at compilation

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scippie



Joined: 26 Jan 2017
Posts: 6
Is it possible to get the file size of a file at compilation
I like to use FASM for my own OS development. Not only do I like how it reminds me of assembler when I was 14 years old when I was coding my first OS in Turbo Assembler, but it also has lots of cool features.

For example, to boot my OS, I create a floppy disk image. I don't need special tools for this, because with the file command, I can just combine different files and with the rb command, I can put them in exact places.

So I first compile all my .asm's and then I join all their binaries by compiling disk.asm which is actually a lot of data definitions and bin-file references.

I am now creating a complete FAT12 structure within that file and it works perfectly. The only problem I have is that in a FAT12 structure, I need to have an exact file size to put in there (at this point, I just always read enough from the disk).

So, like there is a file command, is there something like a filesize function or is there a workaround for this? Or can it maybe be added? It would be so cool to be able to create a complete correct disk image just by compiling my disk.asm file.

I could of course create a script that creates a small .inc file for every .bin file I have, just containing dd <filesize>, but that would become such a mess when the number of files grows.
Post 26 Jan 2017, 19:48
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scippie



Joined: 26 Jan 2017
Posts: 6
I already just thought of a better way, but I'd still prefer a function if there were any.

I just did this:

FOO:
file 'foo.bin'
FOO_end:

and in my FAT12 header:
dw (FOO / 512 - 33 + 2)
dd (FOO_end - FOO)

(I still need to add some constants Smile )
Post 26 Jan 2017, 19:59
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6509
Location: Kraków, Poland
If you use FILE directive to include contents of entire file, you can simply compute the difference in offsets to get the size of that file:

Code:
example file 'example.bin'
example_size = $ - example

And since fasm allows forward-referencing, you can use the value of "example_size" in your headers (before the actual files are included) and fasm is going to resolve these variables so that headers end up containing correct values.

If you do not include entire file anywhere, you can still use the same trick to compute the file size, just use the VIRTUAL block to avoid placing a whole file in the output:

Code:
virtual at 0
    file "example.bin"
    example_size = $
end virtual


The above examples apply to both fasm 1 and fasmg with no difference. But in case of fasmg you can also do some trickier things, like loading the contents of a file into a string:

Code:
; WARNING: this snippet works with fasmg only
virtual at 0
    file 'example.bin'
    load example_string : $ from 0
end virtual

dd lengthof example_string      ; size of file
db example_string               ; contents of file

Post 26 Jan 2017, 20:01
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scippie



Joined: 26 Jan 2017
Posts: 6

Tomasz Grysztar wrote:
If you use FILE directive to include contents of entire file, you can simply compute the difference in offsets to get the size of that file:

Code:
example file 'example.bin'
example_size = $ - example



That's even better! Thanks!


Tomasz Grysztar wrote:
The above examples apply to both fasm 1 and fasmg with no difference. But in case of fasmg you can also do some trickier things, like loading the contents of a file into a string:

Code:
; WARNING: this snippet works with fasmg only
virtual at 0
    file 'example.bin'
    load example_string : $ from 0
end virtual

dd lengthof example_string      ; size of file
db example_string               ; contents of file



Cool stuff. Fasm is great Smile
Post 26 Jan 2017, 20:57
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scippie



Joined: 26 Jan 2017
Posts: 6
So, after reading your text, I wonder if it's possible to have the length of a text string as first byte before the string. Something like:

label db <len>,'Hello world!'
How could <len> be automated?
Post 26 Jan 2017, 21:22
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6509
Location: Kraków, Poland
With fasmg it can be as simple as:

Code:
struc str value
        . db lengthof valuevalue
end struc

hello str 'Hello world!'

With fasm 1 it requires a little bit more work:

Code:
struc str value
{
        local ..length
        . db ..length,value
        ..length = $ - (.+1)
}

hello str 'Hello world!'

Post 26 Jan 2017, 21:27
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scippie



Joined: 26 Jan 2017
Posts: 6
Wow, thanks! As I am using fasm 1, it's the struc.

I had to create an extra struc for strings ending with CR/LF, because the struc wouldn't recognize the extra 13,10 after str as extra data in the string.

Or is there a way to make it work with the same struc? I searched the manual for this (it has been very long since I looked at the manual) but couldn't find it.
Post 26 Jan 2017, 22:22
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l_inc



Joined: 23 Oct 2009
Posts: 875
scippie
You can put an ampersand directly after the argument name in the first line. This will tell fasm to use the whole remainder of an argument line as the last argument. Aside from that passing arguments surrounded by < and > allows to ignore commata as argument separators.

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Post 26 Jan 2017, 22:34
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scippie



Joined: 26 Jan 2017
Posts: 6

l_inc wrote:
scippie
You can put an ampersand directly after the argument name in the first line. This will tell fasm to use the whole remainder of an argument line as the last argument. Aside from that passing arguments surrounded by < and > allows to ignore commata as argument separators.


Thanks! That's perfect.

Is it documented? I didn't see it in the docs, but I'm a terrible reader.

Thanks for all the quick help guys. I feel welcome already!
Post 26 Jan 2017, 23:23
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l_inc



Joined: 23 Oct 2009
Posts: 875
scippie
Yes, it's mentioned in the chapter 2.3.3 Macroinstructions. The manual is quite concise about many things. So they are admittedly easy to miss and sometimes hard to understand.

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Faith is a superposition of knowledge and fallacy
Post 26 Jan 2017, 23:38
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