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MOROS Filesystem

Hard drive

A hard drive is separated in blocks of 512 bytes, grouped into 4 areas:

+------------+
| Boot       | (8192 blocks)
+------------+
| Superblock | (2 blocks)
+------------+
| Bitmap     | (n blocks)
+------------+
| Data       | (n * 512 * 8 blocks)
+------------+

The first area contains the bootloader and the kernel, the second is a superblock with a magic string to identify the file system, the third is a bitmap mapping the allocated data blocks of the last area.

A location on the tree of dirs and files is named a path:

  • The root dir is represented by a slash: /
  • A dir inside the root will have its name appended to the slash: /usr
  • Subsequent dirs will append a slash and their names: /usr/admin

Creation with QEMU

$ qemu-img create disk.img 128M
Formatting 'disk.img', fmt=raw size=134217728

Setup in diskless console

During boot MOROS will detect any hard drives present on the ATA buses, then look for a filesystem on those hard drives. If no filesystem is found, MOROS will open a console in diskless mode to allow the user to create one with the disk format command:

> disk format /dev/ata/0/0

This command will format the first disk on the first ATA bus by writing a magic string in a superblock, mounting the filesystem, and allocating the root directory.

The next step during setup is to create the directory structure:

> write /bin/           # Binaries
> write /dev/           # Devices
> write /ini/           # Initialisation files
> write /lib/           # Libraries
> write /net/           # Network
> write /src/           # Sources
> write /tmp/           # Temporary files
> write /usr/           # User directories
> write /var/           # Variable files

Then the following should be added to the boot script with the command edit /ini/boot.sh to allow MOROS to finish booting:

user login
shell

Finally a user can be created with the following command:

> user create

All of this can be made more easily by running the install command instead. This installer will also add additional files contained in the dsk repository of the source code, like a nice login banner :)

Data Structures

BlockBitmap

Bitmap of allocated blocks in the data area.

Block

A block is small area of 512 bytes on a hard drive, and it is also part of linked list representing a file or a directory.

The first 4 bytes of a block is the address of the next block on the list and the rest of block is the data stored in the block.

Structure:

 0
 0 1 2 3 4 5 6      n
+-+-+-+-+-+-+-+ // +-+
| addr  | data       |
+-+-+-+-+-+-+-+ // +-+

n = 512

Superblock

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2    n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+
| signature     |v|b| count | alloc | reserved       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+

signature = "MOROS FS"
v = version number of the FS
b = size of a block in 2 ^ (9 + b) bytes
count = number of blocks
alloc = number of allocated blocks

File

The first block of a file contains the address of the next block where its contents is stored and the beginning of its contents in the rest of the block.

If all contents can fit into one block the address of the next block will be empty.

Structure:

 0
 0 1 2 3 4 5 6 7 8      n
+-+-+-+-+-+-+-+-+-+ // +-+
| addr  | contents       |
+-+-+-+-+-+-+-+-+-+ // +-+

n = 512

Dir

The first block of a directory contains the address of the next block where its directory entries are stored and the first entries in the rest of the block.

If all entries can fit into one block the address of the next block will be empty.

Structure:

 0                   1
 0 1 2 3 4 5 6 7 8 9 0                            n
+-+-+-+-+-+-+-+-+-+-+-+ // +-+-+-+-+-+-+-+-+ // +-+
| addr  | dir entry 1        | dir entry 2        |
+-+-+-+-+-+-+-+-+-+-+-+ // +-+-+-+-+-+-+-+-+ // +-+

n = 512

DirEntry

A directory entry represents a file or a directory contained inside a directory. Each entry use a variable number of bytes that must fit inside the data of one block. Those bytes represent the kind of entry (file or dir), the address of the first block, the filesize (max 4GB), the last modified time in seconds since Unix Epoch, the length of the filename, and the filename (max 255 chars) of the entry.

Structure:

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4      m
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+
|k| addr  | size  | time          |n| name buffer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+

k = kind of entry
n = length of name buffer
m = 17 + n

FileInfo

The info syscall on a file or directory and the read syscall on a directory return a subset of a directory entry for userspace programs.

Structure:

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0      m
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+
|k| size  | time          |n| name buffer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // +-+

k = kind of entry
n = length of name buffer
m = 13 + n