File systems and file formats

A file system is the method an operating system uses to organise data on a storage device into named files and directories, and to keep track of where each file's contents physically reside. It manages free space, records file names and attributes, and lets programs read and write data without dealing directly with the raw layout of the disk. PCs have used a succession of file systems over the years, and the choice of file system affects features such as the maximum file size, naming rules, and reliability.

Before any file system can be used, the computer must start up. The boot process begins when firmware on the motherboard tests the hardware and then loads a small program from a designated boot device, such as a hard disk or a USB key, which in turn loads the operating system. Making a device bootable involves writing the right startup code and arranging the partitions so the firmware can find and run it.

Files are also distinguished by their format, the internal structure that determines how their bytes should be interpreted. Among the most common are image formats. GIF supports a limited palette and simple animation and uses lossless compression; JPEG uses lossy compression well suited to photographs, trading some detail for much smaller files; and PNG offers lossless compression with support for transparency, making it a popular choice for graphics and screenshots. Knowing a file's format is essential, because the same bytes mean nothing without the rules for reading them.

Frequently asked questions

What is a file system?

A file system is the way an operating system organises data on a storage device into files and directories and tracks where each file's contents are stored.

How do JPEG and PNG differ?

JPEG uses lossy compression that suits photographs and produces small files at the cost of some detail, while PNG uses lossless compression and supports transparency, which suits graphics and screenshots.

What makes a disk or USB key bootable?

A bootable device contains startup code that the computer's firmware can locate and run, which then loads the operating system; preparing one involves writing this code and arranging the partitions correctly.