The layout of Filesystem on a disk is composed of four consecutive logical parts:

• The Superblock is the very first block of the disk and contains information about the file system (number of blocks, size of the FAT, etc.)
• The File Allocation Table is located on one or more blocks, and keeps track of both the free data blocks and the mapping between files and the data blocks holding their content.
• The Root directory is in the following block and contains an entry for each file of the file system, defining its name, size and the location of the first data block for this file.
• Finally, all the remaining blocks are Data blocks and are used by the content of files.

The size of virtual disk blocks is 4096 bytes.

Unless specified otherwise, all values in this specification are unsigned and the order of multi-bytes values is little-endian.

Superblock

The superblock is the first block of the file system. Its internal format is:

Offset	Length (bytes)	Description
0x00	8	Signature (must be equal to "ECS150FS")
0x08	2	Total amount of blocks of virtual disk
0x0A	2	Root directory block index
0x0C	2	Data block start index
0x0E	2	Amount of data blocks
0x10	1	Number of blocks for FAT
0x11	4079	Unused/Padding

If one creates a file system with 8192 data blocks, the size of the FAT will be 8192 x 2 = 16384 bytes long, thus spanning 16384 / 4096 = 4 blocks. The root directory block index will therefore be 5, because before it there are the superblock (block index #0) and the FAT (starting at block index #1 and spanning 4 blocks). The data block start index will be 6, because it's located right after the root directory block. The total amount of blocks for such a file system would then be 1 + 4 + 1 + 8192 = 8198.

FAT

The FAT is a flat array, possibly spanning several blocks, which entries are composed of 16-bit unsigned words. There are as many entries as data blocks in the disk.

The first entry of the FAT (entry #0) is always invalid and contains the special EOC (End-of-Chain) value which is 0xFFFF. Entries marked as 0 correspond to free data blocks. Entries containing a positive value are part of a chainmap and represent a link to the next block in the chainmap.

Note that although the numbering in the FAT starts at 0, entry contents must be added to the data block start index in order to find the real block number on disk.

The following table shows an example of a FAT containing two files:

• The first file is of length 18,000 bytes (thus spanning 5 data blocks) and is contained in consecutive data blocks (DB#2, DB#3, DB#4, DB#5, DB#6 and DB#7).
• The second file is of length 5,000 bytes (this spanning 2 data blocks) and its content is fragmented in two non-consecutive data blocks (DB#1 and DB#8).

Each entry in the FAT is 16-bit wide.

FAT index:	0	1	2	3	4	5	6	7	8	9	10	...
Content:	0xFFFF	8	3	4	5	6	7	0xFFFF	0xFFFF	0	0	...

Root directory

The root directory is an array of 128 entries stored in the block following the FAT. Each entry is 32-byte wide and describes a file, according to the following format:

Offset	Length (bytes)	Description
0x00	16	Filename (including NULL character)
0x10	4	Size of the file (in bytes)
0x14	2	Index of the first data block
0x16	10	Unused/Padding

An empty entry is defined by the first character of the entry's filename being equal to 0.

The entry for an empty file, which doesn't have any data blocks, would have its size be 0, and the index of the first data block be FAT_EOC.

Continuing the previous example, let's assume that the first file is named "test1" and the second file "test2". Let's also assume that there is an empty file named "test3". The content of the root directory would be:

Filename (16 bytes)	Size (4 bytes)	Index (2 bytes)	Padding (10 bytes)
test1\0	18000	2	xxx
test2\0	5000	1	xxx
test3\0	0	FAT_EOC	xxx
\0	xxx	xxx	xxx
...	...	...	...

Reference program and testing

• Create a new virtual disk containing an empty file system: $ filesystem.x make <diskname> <data block count>
• Get some information about a virtual disk: $ filesystem.x info <diskname>
• List all the files contained in a virtual disk: $ filesystem.x ls <diskname>
• Etc. in order to have the list of commands: $ filesystem.x