With the Windows 95 installable file system, multiple file systems can coexist on the computer. Windows 95 includes the following file systems.
VFAT.
In Windows 95, the 32-bit virtual File Allocation Table file system is the primary file system and cannot be disabled. VFAT can use 32-bit, protected-mode drivers or 16-bit real-mode drivers. Actual allocation on disk is still 12-bit or 16-bit (depending on the size of the volume), so FAT on the disk uses the same structure as previous versions of this file system. VFAT handles all hard disk drive requests, using 32-bit code for all file access for hard-disk volumes. VFAT was first introduced in Windows for Workgroups version 3.11 as an optional FAT file system that processed file I/O in protected mode.
CDFS.
The virtual CD-ROM file system has the same responsibilities for a CD-ROM device as VFAT has for a standard hard disk. If a CD-ROM device is detected, the CDFS driver loads dynamically. When CDFS is installed, the standard disk type-specific device and Disk SCSI translator are replaced with CD-ROM versions. The CDFS driver is a protected-mode version of MSCDEX.EXE, providing the interface from the CD-ROM device to the operating system, as described later in this chapter and in Chapter 31, "Windows 95 Architecture."
If the CD-ROM drive and its drivers support the multisession command, CDFS can support multisession capabilities, which provide a method for adding data to a CD-ROM (this is most applicable to CD-Recordable media). The multisession command returns a number that identifies the first sector of the last session on the media so that CDFS can recognize the media.
Network redirectors.
A network redirector (such as Microsoft Client for NetWare Networks or Client for Microsoft Networks) is a file system driver that accesses the network file system. Windows 95 supports multiple network redirectors simultaneously, as described in Chapter 32, "Windows 95 Network Architecture."
All these file systems support long filenames and can use the protected-mode cache (VCACHE) for read-ahead. VFAT also supports lazy-write throughput, so applications can write immediately to the cache, and VFAT can write the information to disk later. For more information, see "VCACHE and CDFS Supplemental Cache" later in this chapter.
Other software vendors can also implement file systems. For example, a vendor might provide a file system that allows a computer running Windows 95 to connect to a different operating system (for example, Apple® Macintosh® or UNIX®) to share files.
The Installable File System Manager (IFSMGR) receives all INT 21 calls and determines which file system driver should receive the call to process it. IFSMGR uses a real-mode stub named IFSHLP.SYS to send INT 21 calls back to IFSMGR, as described in Chapter 31, "Windows 95 Architecture."
File system drivers manage the high-level I/O requests from applications. The file system driver processes requests from applications and initiates low-level I/O requests through the I/O Supervisor.
Protected-mode disk compression is not integrated into the file system, but is supported by a layer in the I/O subsystem. Windows 95 supports disk compression software created for earlier versions of MS-DOS, using their real-mode driver loaded from CONFIG.SYS.