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Concept
Installable File System
Summary
The Installable File System (IFS) is a in MS-DOS/PC DOS 4.x, IBM OS/2 and Microsoft Windows that enables the operating system to recognize and load drivers for s.
When IBM and Microsoft were co-developing OS/2, they realized that the did not offer some of the features modern OSes would require, and Microsoft began developing the (HPFS), codenamed Pinball.
Instead of coding it inside the kernel, as FAT was, Microsoft developed a "driver-based" filesystem API that could allow them and other developers to add new filesystems to the kernel without needing to modify it.
When Microsoft stopped working on OS/2, IBM continued using the IFS interface and Microsoft implemented a similar one in Windows NT.
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 up), 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.
Official source
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