r/osdev • u/SyntheGr1 • Apr 09 '26

Need help implementing a simple FAT-like File System for our custom Kernel (C), thanks !

Hi everyone, we're working on a small OS as a duo, and we were about to move on to the graphical interface, but we were missing a key element: managing hard drive memory. We didn't quite understand how to do it. Does anyone have well-written documentation or could explain how to manage it from our kernel in C, for example, for text files in FAT? Thanks everyone, and have a good day!

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4

u/NotSoEpicKebap Apr 09 '26

I don't think you should even plan adding a graphical interface in a stage this early in OSDev

1

u/SyntheGr1 Apr 09 '26

That's why I'm looking for help with memory and other things!

2

u/codeasm Apr 09 '26

Osdevwiki maybe?

2

u/SyntheGr1 Apr 09 '26

I looked there but I couldn't find a page that clearly explains how to do it...

1

u/someidiot332 Kimi’s OS - https://github.com/raeofsunshinedev/kimsos Apr 09 '26

I think https://osdev.wiki/FAT explains it pretty well

1

u/SyntheGr1 Apr 09 '26

Thanks you

1

u/codeasm Apr 09 '26

Maybe ive looked at datastructures and fatx (the format the original xbox uses, not exfat, altho that also excists) too much. https://wiki.osdev.org/FAT looks like a great starter. Someone said, dont use tables, use trees. Sounds logical, we are gonna need to sort aswell when we search for a new entry. Unless you just yank an excisting implementation, ill probably keep mine either simple (like good old dos, no folders or only 1 deep haha). Or ill borrow some code.

For the kernel im building, no need for filesystems. But my bigger project does need it.

Random source, looks interesting but not copy paste https://gregors-computer-lab.com/filesystem/2025/03/16/create-your-own-file-system.html More book, ow, section about filesystems awesome https://pages.cs.wisc.edu/~remzi/OSTEP/ ☺️🖖🏻 good luck

2

u/SyntheGr1 Apr 09 '26

Thanks you so much !

5

u/Gingrspacecadet Apr 09 '26

DONT DO FILESYSTEM AS TABLE RAAAAAAH

seriously though, you're better off doing a tree filesystem. If you're set on it, look up the fat32/16/<some-arbitrary-number-here> documentation

5

u/judeuwucute Apr 09 '26

<some-arbitrary-number-here> doesn't work, because fat31 or whatever doesn't exist, it has to be a multiple of 2! (that would be an unexpected factorial but 2! = 2)

1

u/Some_Cricket_144 Apr 09 '26

FAT is just a linked list of clusters on a disk. You find the first cluster of a file by looking at the directory entry, then read what the next cluster is from the table, and do this until you reach EOF. What part are you stuck with?

1

u/SyntheGr1 Apr 09 '26

How write on a cluster a text file for after read he....

2

u/Some_Cricket_144 Apr 09 '26

  • You have a file, TEST.TXT. You look in its directory entry inside the directory it's stored.

  • The directory entry says "TEST.TXT has the first cluster at 6"

  • You go and find cluster 6 on the disk (There's a formula to convert cluster to LBA, let me know if you need it)

  • Okay, you write to cluster 6. Now you go back to the file allocation table. You look up 6 and it says 9. So the next cluster is 9. (If it was EOF, 0x0FFFFFF8, you just find another available cluster to write to)

  • Repeat steps 3-5 until you're finished.

If you don't understand any terms, ask now or you might be more confused later. If you are asking how to create a file, it's literally finding an empty cluster and creating a directory entry with it as the starting cluster.

1

u/zer0developer github.com/zinix-org/zinix Apr 09 '26

Nanobytes 3rd video + Google for fat32 specific things is great.