What Format is a Floppy Disk?

Floppy disks, also known as floppy drives, have been a staple of computing for decades. They were used to store and transfer data between computers, and many people still use them today. But did you know that not all floppy disks are created equal? In this article, we'll explore the format of a floppy disk, including what makes it bootable.

When a computer boots from a floppy disk, the BIOS (Basic Input/Output System) plays a crucial role in loading the operating system or program into memory. The process begins when the BIOS reads the first sector of the first track on the first side of the disk, which is typically 512 bytes long. This sector contains important information that helps the BIOS determine how to load the rest of the disk.

One key component of a bootable floppy disk is the FAT12 header, which occupies the space between bytes 3 and 30. This header provides essential data structures for the file system, allowing the BIOS to navigate the disk and load files. The BIOS then jumps to address 0000:7c00h, where it expects to find a jump instruction that leads to the actual code.

In some cases, the BIOS may also perform checks on the disk before booting it. For example, it may verify that the last two bytes of the sector are 55h AAh, which is known as the 'boot signature.' If this signature is missing or incorrect, the BIOS will ignore the disk and attempt to boot from another source.

Writing a useful bootable floppy disk requires an understanding of what the BIOS does when it boots from a floppy. It's essential to recognize that almost all floppy disks are capable of being booted, but they may not always contain a program that is especially useful. In many cases, floppies will simply display a message asking you to insert a system disk or execute INT 18h, which runs BASIC if it's installed in ROM.

In conclusion, the format of a floppy disk is crucial for bootability. By understanding what the BIOS does when it boots from a floppy, you can create a useful and functional bootable disk that meets your needs. Whether you're an IT professional or simply a tech enthusiast, having a grasp on the basics of floppy disk formatting can be incredibly valuable.

What format is floppy disk