.ADF

Introduction

An ADF file is a copy of a Commodore Amiga floppy disk. Amiga emulators load ADF files in place of real disks, and most of the Amiga’s software survives today in this form.

An ADF holds the decoded data of every sector, in order. A standard double density disk gives a file of exactly 901,120 bytes, and a high density disk twice that. The format does not keep the raw signal from the disk, so copy-protected disks that rely on unusual tracks need a fuller format to work.

Software

• WinUAE, an Amiga emulator (Windows)
• FS-UAE, an Amiga emulator (Windows, macOS, Linux)
• Amiga Forever, a packaged Amiga emulation suite (Windows)
• amitools, whose xdftool reads and writes ADF files (Windows, macOS, Linux)

Structure

A double density disk has 80 cylinders, 2 heads and 11 sectors of 512 bytes per track, 1,760 blocks in all. The ADF stores them in order: cylinder 0 head 0, cylinder 0 head 1, cylinder 1 head 0 and so on.

The first two blocks are the boot block. It starts with the letters DOS and a flag byte giving the file system: 0 for the original file system (OFS), 1 for the fast file system (FFS), with higher values adding international characters and directory caching. A checksum and boot code follow. The root block sits in the middle of the disk, at block 880, and holds the disk name and a hash table that points to the files and directories at the top level. Every file has a header block listing its data blocks. OFS data blocks start with a 24-byte header, while FFS data blocks hold only data.