IFS in OS/2
The IFS provided a basic and powerful interface for programming filesystems. It was introduced in 1989 in OS/2 1.20, along with the HPFS filesystem.
Filesystem drivers executed in kernel-space (ring 0) and are divided in four principal pieces: microIFS, miniIFS, IFS, helpers.
Only the IFS and the filesystem code itself is required and it is loaded via an "IFS=" statement in the CONFIG.SYS file. It is a NE 16-bit dynamically loaded library. No matter if it is a 32-bit OS/2 (2.0 and newer), the IFS is always 16-bit (although extraofficially you can make a 32-bit IFS).
The microIFS is a piece of code that loads in memory the kernel and the miniIFS and jumps to kernel execution. It is usually in the boot portion of the filesystem.
The miniIFS is a piece of code that is called by the kernel to load the first IFS statement that appears in the CONFIG.SYS file, so the first IFS statement must be the boot's filesystem for the system to be able to boot.
The helpers are 16-bit (for OS/2 1.x) or 32-bit (for OS/2 2.x and upper), are executed in user-space (ring 3) and contain the code used for typical filesystem maintenance, and are called by CHKDSK and FORMAT utilities.
This four-piece scheme allowed developers to dynamically add a new bootable filesystem, as the ext2 driver for OS/2 demonstrated.
CD-ROM filesystem driver (ISO 9660) was added in OS/2 2.0, UDF was added in OS/2 4.0 and JFS was added in OS/2 4.5. eComStation, the latest packaging of OS/2, also includes many filesystem drivers for OS/2 in its companion CDs. There was also an official 32-bit HPFS IFS, called HPFS386 that improved performance and added some features, like variable size cache and Access Control Lists, and was available only in OS/2 3.0 server edition. The FAT filesystem was never removed from the kernel and officially never an IFS, although there are FAT IFS that added features like long file names (LFNs), FAT32 support, etc.
Network file-sharing protocols like NFS and SMB are also implemented using IFS, and the IFS interface never changed.
Read more about this topic: Installable File System