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Hi,
I've got a few "problems".

this code creates a big binary, 'cause I set c to 5.
gcc adds a lot of "add [eax],al" commands to my code instead of using the code segment, how do I change that?
(a c code without char c=5; function works fine...)

I'm using this compile line:
Also,
how do I create a file system?
for example:

I wrote an header which can write and read sectors of the disk,
so how do I open file?...
the files start at the end of the disk,
if you read a file, how does it know which disk it should use,
and how does it get the sector data from the disk? (the filesytem)

frank wrote:gcc adds a lot of "add [eax],al" commands to my code instead of using the code segment, how do I change that?
(a c code without char c=5; function works fine...)

Note that "add [eax], al" equates to a string of zero bytes. Which is normal: you're going to have 05 followed by (padding-1) bytes of zeroes, where padding is the section alignment. You can't avoid that. Padding the data section with zeroes is completely normal.

I wrote an header which can write and read sectors of the disk,
so how do I open file?...
the files start at the end of the disk,
if you read a file, how does it know which disk it should use,
and how does it get the sector data from the disk? (the filesytem)

A disk contains some raw data in some format: think of it as an array of, say, 4 gigabytes. You've got to translate that data somehow, which is where the file system comes in. First read a boot on OS development to get the general idea (e.g. OS: D&I by Tanenbaum), then read some real file system specs (e.g. FAT or ext2 specs on http://www.nondot.org/sabre/os/).

A very simple filesystem you can imagine : you have at a well-known place a sector containing the directory of your disk : n entries having a symbolic name, a first sector, a sector count and a byte count.

You need to lookup the directory for the file's name (open operation) and usually store the lookup result (size & first sector) in a structure that you will use as file handle.
Then if you want to access datas at ith byte of your file, you just read first_sector + (i mod sector_size) and get the wanted byte out of it (well, a small cache of 1 or 2 sectors per file handle will quickly become your friend

In this scheme, it's quite hard to extend files or to remove them because you have nothing to keep track of freed sectors (allocation must be done top-down or bottom-up so that you're sure to have contiguous sectors for your file) but this is no problem for a boot partition that is rather static...