rohankumardubey / file-system-simulation

FAT File System Simulation

Introduction

This application is a simulation of the File Allocation Table (FAT) file system, programmed in Java. There are a total of 5 files in the application. Each file performs different functions of a file system, on a single common file, which acts as a virtual hard disk.

FAT File System

The FAT file system, irrespective of the particular specification of it such as FAT32 or FAT16, divides the the available main memory into 3 areas, namely,

• Boot Sector
• Meta Data Area
• File Data Area

The first is the Boot sector; this section contains all the intialization functions and variables where the entire sturcture of the file system is defined, such as the number of clusters and the size of each cluster. The next section is the meta data section, where the meta data of all the files that are created is stored. This includes the size of the file, extension and date of creation, among others. This unit also stores the actual file allocation table, used to look up the location of a file in the third unit, which contains the actual data of the files stored. This data unit is divided into clusters of certain sizes. These clusters are a logical construct that are used to store data.

Simulating the FAT File System On a File

For the simulation, we first need some empty memory to work on. We can create a text file of a specific size. We fill the file with a special character to ensure that the file of desired size has been created and is not empty. We now use this file as a virtual hard disk to carry out all the operations of a file system. These functions include

• Create File
• Delete File
• Read File
• Edit File
• Move File
• Rename File

Implementation

We first create a file that is 2 MBs in size. To ensure we do not create an empty file, we fill this file with the ‘#’ character enough times, so that the file size is 2 MBs. We then divide this 2 MB file into the 3 units described above. We allocate 8192 bytes to the boot or initialization unit. When you run the initialization file, it overwrites the # characters with the structure of the file system, i.e, how many bytes have been allocated to the different units, how many clusters there are and the size of each cluster. We have created 4 clusters and we have imposed a restriction that no more than 4 files can be created in this file system.

We make use of Java’s file handling operations to carry out the specifix tasks. Specifically, we make use the RandomAccessFile class. Using this class allows us to move freely in the file and make changes in a specific part of the file while retaining the contents of any other part.

To implement the FAT file lookup procedure, we use the RandomAccessFile’s seek method. The seek method gives us a pointer to that specific byte and we can jump to it. For example, if we create an object this class named file, we can jump to the 200th byte as so

file.seek(200)

This allows us to make distinctions between the different areas in our virtual hard disk. When we ru nthe application, it reads the number of files that have been created already from the boot sector or initialisation area. If there are 4 files created and the user wants to create another file, it throws an exception. If there are some number of files created that are valid and we want to run any operations on that file, we jump to the starting pointer of that file and make any changes we want to.

Files

• MemoryCreateAndInitialize file creates the 2 MB file filled with #'s
• MemoryIntialize file writes the intialization details in the first 8192 bytes of the file
• FileAccessAndUse file contains all the methods for creating, deleting, updating, renaming, reading and writing data onto the file.
• FileAccessAndUseInterface file is a continously running file that prompts the user to provide input to perform any of the operations given above.