Introduction

XFS or eXperimental File System is a file system architecture designed for XOS (eXperimental Operating System). XFS is a simple filesystem which has no directory structure.
XFS spans the entire disk in XSM (eXperimental String Machine). The disk consists of a linear sequence of 512 blocks. The basic unit of storage in XSM is a block. The size of the block is equal to that of page in memory (512 words). The total capacity of the disk is 512 * 512 = 262144 words.
Any particular block in the disk is addressed by the corresponding number in the sequence 0 to 511 known as the block number.

Disk Organization

The disk is organized by the file system as shown below.

• OS Startup Code: Block 0 is the location of operating system code required during machine boot.

• Exception and Interrupt Handler: Blocks from 1 - 18 are intended for Interrupt Routines for interrupt and exception handling which is done by the Operating System.

• FAT or File Allocation Table contains details about the files stored on the disk.

• Disk Free List has 512 entries, one entry for each block in the disk, indicating whether it is used or unused.

• INIT Code It has the code for INIT process, which is the first user program run by the OS after startup.

• User Blocks: Blocks from 24 to 447 are used to store user data files and user programs.

• Swap Area: The file system also provides a swap area for the operating system to implement demand paging. Swap Area is reserved exclusively for use by the operating system.

File

A file is a collection of blocks identified by a name. Every file in the disk has a Basic Block and several Data Blocks. They are defined as follows:

• Basic Block: It consists of information about the data in a file.

• Data Blocks: These blocks contain the actual data of a file.

The structure of the basic block is given below.

Index	0--255	256--511
Content	Block List	Header



• Block List: It contains block addresses of all data blocks in the file.
  • The block list consists of 256 entries.
  • Each entry is of size one word.
  • The value contained in an entry of the block list gives the block number of the corresponding data block in the disk. All invalid entries are marked with -1.
• Header: The header contains meta information related to the file. The header field is 256 words long. It can be used to store details like permissions, ownership, etc related to a file, similar to inode in Linux and UNIX operating systems. However, XOS does not use the header field.

Consider the example illustrated by figure 3. From the figure, we infer the following:

• The zeroth data block of the file resides in the disk in block number 240
• The first data block of the file resides in the disk in block number 420. The second data block of the file resides in the disk in block number 101. The third data block of the file resides in the disk in block number 315
• There are no more data blocks for the file. So rest of the entries of the basic block are marked as -1.

File Types

Data files

Executable files

File Allocation Table (FAT)

• Total size of the FAT is thus 512 words, which occupies 1 block.
• It is a disk data structure and occupies block number 19 on the disk.

Structure of FAT

Disk Free List

• The Free List of the disk consists of 512 entries. Each entry is of size one word.
• The size of the free list is thus 1 block or 512 words.
• It is present in block 20 of the disk. Refer Disk Organization.
• For each block in the disk there is an entry in the Disk Free List which contains a value of either 0 or 1 indicating whether the corresponding block in the disk is free or used, respectively. Each entry in the Disk Free List is of size one word.
• Blocks 0 to 20 are system reserved and are marked as 1 in the Disk Free List so that they cannot be used for saving user files. The file system also ensures that user files are not stored on the Swap Area.