A virtual fast disk device allows for direct, asynchronous I/O to hard drives or other block devices such as optical disks or tape backup devices. This virtual device improves performance by supporting such features as true read-ahead and write-behind paging and file caching. Also, since a fast disk device can be called at ring 0, MS-DOS programs running in virtual machines can be paged while they run.
Every fast disk device relies on the virtual block device (BLOCKDEV) to provide a common interface between the fast disk device and its clients. In a standard Windows configuration, the fast disk device has two clients: the virtual Interrupt 13h device (INT13) and the virtual swap file device (PAGEFILE). These clients send commands to the fast disk device to read and write sectors on the disk.
All fast disk devices use the services and structures of the virtual block device. For complete information about these services, see Chapter 4, “Block Devices.”