Mixed Kernel-driver-based API
In this API all filesystems are in the kernel, like in kernel-based APIs, but they are automatically trapped by another API, that is driver-based, by the OS.
This scheme was used in Windows 3.1 for providing a FAT filesystem driver in 32-bit protected mode, and cached, (VFAT) that bypassed the DOS FAT driver in the kernel (MSDOS.SYS) completely, and later in the Windows 9x series (95, 98 and Me) for VFAT, the ISO9660 filesystem driver (along with Joliet), network shares, and third party filesystem drivers, as well as adding to the original DOS APIs the LFN API (that IFS drivers can not only intercept the already existent DOS file APIs but also add new ones from within the 32-bit protected mode executable).
However that API was not completely documented, and third parties found themselves in a "make-it-by-yourself" scenario even worse than with kernel-based APIs.
Read more about this topic: File System API
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