Direct Access

Applications typically use direct access to make changes to the directory entries, paths, and allocation chains in the file allocation table (FAT) of a given volume. The applications access this information by using the Interrupt 13h functions, Absolute Disk Read and Write (Interrupts 25h and 26h), or the Interrupt 21h read, write, and format track IOCTL functions. Some applications may also access the volume using the disk controller input and output (I/O) ports.

As a multitasking operating system, Windows 95/98 permits any number of applications to access a volume at a given time. Applications that change the file system structures of a volume without regard to other applications risk corruption of that information and subsequent data loss. To prevent data loss, the system manages all requests for direct access. With the exception of floppy disk drives, Windows 95/98 does not permit direct write operations, unless the volume has been locked by the application. The system returns the ERROR_WRITE_PROTECT error value to functions or interrupts that attempt direct write operations when the volume has not been locked. An application can read directly from a volume, but the system may satisfy the request by reading from internal caches rather than from the medium itself. An application cannot access disk controller I/O ports; the system traps all access.

An application can override the default behavior of the system by requesting exclusive use of the volume using the lock and unlock volume IOCTL functions. An application that has exclusive use of a volume can read and write directly to the medium, and because the system flushes internal caches to the medium, the information there reflects the actual state of the volume. Locking a volume (including the floppy disk drives) ensures consistency of data, because other processes cannot update information about the volume while it is locked.