How to implement a filesystem

[deleted]

Ok, so thanks to a few people, I now have a compiled, linked, coded and running operating system. Features include: working GDT, working IDT and IRQ hander, a working VGA driver, a working Keyboard driver, a simple set of string function (thanks to gcc), and a simple shell.
The missing thing: a working filesystem.

So my question, how to I go from importing bytes to reading files. Any help would be awesome!!

Thanks

[alexfru]

Functional GDT and IDT are not OS features.
Nor are string subroutines, unless, perhaps exposed to applications through an API.

You probably need to read about file systems first. There are books and articles on the subject. You may want to read about a simple specific FS (e.g. FAT) first to put things into context. Google up!

[Viperidae]

Well first comes the virtual filesystem which would serve as an abstraction layer for your actual file system(a). You could do this the Windows/DOS way (Each drive has its own label, IE C:, A:, B:, ect) or the Unix way which consists of a root filesystem which you can mount other filesystems on. You also will want to have some abstraction layer for storage devices too (This way you can write a filesystem driver for a certain filesystem, and have it work across various devices). The OSDev wiki has some pretty good articles relating to filesystem theory, I suggest you do some more research into the subject.

[embryo]

The missing thing: a working filesystem.

First you need an HDD driver. Or if you prefer to read from USB-flash then you need a USB driver. After it you need a library, that maps file-system data structures to something meaningful for you. After you can map raw bytes of a chosen file-system to something meaningful you can connect HDD driver to the mapping library in such a way that HDD driver can provide your library with raw bytes. Next you need to create some interface for an OS user to be able to use your file-system. It may be a UI or program interface for another library. After it you have a file-system.

[Evans]

Embryo how do you make it so that the hdd provides the library with raw bytes?

[deleted]

Ok thanks guys. I have read a lot of the articles on the wiki, but they do not show how to read a byte from a file, create a new file using the methods. I am planning to use a Fat32 system. Does anyone have any tutorials or implementation of how to create a file on Fat32? I haven't idea.

Please note I am not a newbie programmer, I just don't know much about filesystems.

[Bender]

Microsoft had released FAT specifications long ago. Google for "fatgen103.pdf". There are no such tutorials for filesystems because they are design specific and don't confuse filesystems with disk-based filesystems. Filesystems are just a way of representing data in an organized manner (usually in a tree). Even a device (e.g. a printer, or a terminal) can be seen as a file and can be a part of a filesystem.
Perhaps this should also help http://www.maverick-os.dk/FileSystemFor ... ystem.html
Yeah, I know, UNIX ideology. lol

[iansjack]

Please note I am not a newbie programmer, I just don't know much about filesystems.

In that case, could I suggest that you might find it more constructive, and more satifying, to do a little Googling and reading rather than asking others to do your research for you? This is not rocket science and there is a ton of information, even whole books on the subject, on the web.

It doesn't bode well for your chances of success if you have to turn to others every time you come across a simple matter like this. Things are going to get a lot more complicated down the line.

[deleted]

In that case, could I suggest that you might find it more constructive, and more satifying, to do a little Googling and reading rather than asking others to do your research for you? This is not rocket science and there is a ton of information, even whole books on the subject, on the web.

It doesn't bode well for your chances of success if you have to turn to others every time you come across a simple matter like this. Things are going to get a lot more complicated down the line.

I have googled quite a lot. I just don't understand, how do you create a file in my operating systems code. Can I just use fopen? Or do I need to write these my own using the FAT? These are more my questions.

[MollenOS]

Read the FAT table if you use FAT, find a free FAT entry and allocate it by marking it as used, and voila, you have created a file. Of course you have to create your own fopen function, you also have to make a virtual filesystem, and then implement FAT32 as a filesystem. Read the document as mentioned earlier fatgen103.doc, it will explain everything you ever need to know about FAT32, I used that document to make my FAT32 driver

[deleted]

Ok so I now understand how to create a file with FAT, but how would I: a. access that file from the drive, b. find that file again later?

[MollenOS]

The FAT table contains ALL file records and where they are placed on the harddrive, all the file information you need is in the FAT table. But I still feel like you are not grasping the concept fully, you REALLY need to read up on virtual filesystems and read the document please.

[deleted]

I now understand the concept. But how do I read from the file allocation table? Do I import a byte, return an integer, that kind of thing.

PS: I read throug the docs

[iansjack]

I'm sorry, but you really don't understand the concept. You may wish to read the documentation again, perhaps working out on paper or by experimentation exactly what happens when a file is read or written.

Anyway, one further attempt to explain. Here's how you read a file from a FAT filesystem.

1. You find the directory entry. If the file is in a subdirectory you will have to follow the chain of directories to it; easier to start with a file in the root directory.

2. The directory entry contains a pointer to the first cluster. You read that cluster from the disk.

3. You then read the File Allocation entry for that cluster. It points to the next cluster in the file, or contains an end of file marker. In the latter case you have read the complete file.

4. You read that cluster from the disk and so on until you have read the entire file.

One slight complication is that the sum of all the clusters is almost certainly larger than the file; you only need part of the final cluster. As the directory entry tells you the size of the file that is easily accounted for.

Use a hex editor to examine the various structures on a FAT-formatted disk. Follow those steps manually to see how the file is stored on the disk. Then you will have an understanding how a FAT file system works. You can then try more complicated file systems like ext2, etc. Forget about trying to read from an NTFS file system; Microsoft don't publish details of that. It doesn't matter; there are far better file systems available where you have access to the source code.

[deleted]

Ok thank you, I think I now have a grasp on this.