Media Check (Device-Driver Function 01h)

MEDIAREQUEST STRUC

mrLength db ? ;length of record, in bytes

mrUnit db ? ;unit number

mrFunction db 01h ;function number

mrStatus dw ? ;status

mrReserved db 8 dup(?) ;reserved

mrMediaID db ? ;INPUT: current media descriptor

mrReturn db ? ;OUTPUT: return value

mrVolumeID dd ? ;OUTPUT: previous volume identifier

MEDIAREQUEST ENDS

Media Check (Device-Driver Function 01h) determines whether the medium in the specified drive has changed.

This function is required by block-device drivers only.

Fields

mrLength

Specifies the length, in bytes, of the MEDIAREQUEST structure.

mrUnit

Specifies the unit for which the medium is to be checked.

mrFunction

Specifies the Media Check function: 01h.

mrStatus

Specifies the status of the completed function. If the function is successful, the driver must set the done bit (bit 8). Otherwise, the driver must set both the error and done bits (bits 15 and 8) and copy an error value to the low-order byte.

mrReserved

Reserved; do not use.

mrMediaID

Specifies the media descriptor for the medium MS-DOS assumes is in the drive. Following are the most commonly used media descriptors and their corresponding media:

Value

Type of medium

0F0h	3.5-inch, 2 sides, 18 sectors/track (1.44 MB); 3.5-inch, 2 sides, 36 sectors/track (2.88 MB); 5.25-inch, 2 sides, 15 sectors/track (1.2 MB). This value is also used to describe other media types.
0F8h	Hard disk, any capacity.
0F9h	3.5-inch, 2 sides, 9 sectors/track, 80 tracks/side (720K); 5.25-inch, 2 sides, 15 sectors/track, 40 tracks/side (1.2 MB).
0FAh	5.25-inch, 1 side, 8 sectors/track, (320K).
0FBh	3.5-inch, 2 sides, 8 sectors/track (640K).
0FCh	5.25-inch, 1 side, 9 sectors/track, 40 tracks/side (180K).
0FDh	5.25-inch, 2 sides, 9 sectors/track, 40 tracks/side (360K). This value is also used for 8-inch disks.
0FEh	5.25-inch, 1 side, 8 sectors/track, 40 tracks/side (160K). This value is also used for 8-inch disks.
0FFh	5.25-inch, 2 sides, 8 sectors/track, 40 tracks/side (320K).

mrReturn

Receives a return value identifying whether the medium has changed. The driver must set the field to one of the following values:

Value

Meaning

0FFh	Medium has been changed.
00h	Driver cannot determine whether medium has been changed.
01h	Medium is unchanged.

mrVolumeID

Receives the 32-bit address (segment:offset) of a zero-terminated ASCII string specifying the volume identifier of the previous disk in the drive. The driver must set this field to the address of the volume identifier. If the disk does not have a volume identifier, the driver must set the field to the address of the string “NO NAME”.

Comments

If the medium in the drive has not changed, MS-DOS proceeds with the disk operation.

If the medium in the drive has changed, MS-DOS invalidates all buffers associated with the drive, including any buffers containing data waiting to be written (this data is lost). MS-DOS then calls Build BPB (Device-Driver Function 02h) to request a BPB structure for the new disk and reads the disk's file allocation table (FAT) and directory.

If the driver cannot determine whether the disk has changed, MS-DOS checks its internal disk buffers. If data is waiting to be written to the disk, the system assumes that the disk has not changed and attempts to write the data to the disk. If the disk buffers are empty, MS-DOS assumes the disk has changed and updates the disk information as if the driver had returned 0FFh.