The media ID byte has the following meanings:Obtains selected information about the default disk drive and a pointer to the media identification byte from its file allocation table.
Call with:
AH = 1BH
Returns:
If function successful
AL = sectors per cluster
DS:BX = segment:offset of media ID byte
CX = size of physical sector (bytes)
DX = number of clusters for default drive
If function unsuccessful (invalid drive or critical error)
AL = FFH
Notes:
The media ID byte has the following meanings:
0F0H 3.5-inch double-sided, 18 sectors or "other"
0F8H fixed disk
0F9H 5.25-inch double-sided, 15 sectors or 3.5-inch double-sided, 9 sectors
0FCH 5.25-inch single-sided, 9 sectors
0FDH 5.25-inch double-sided, 9 sectors
0FEH 5.25-inch single-sided, 8 sectors
0FFH 5.25-inch double-sided, 8 sectors
To obtain information about disks other than the one in the default drive, use Int 21H Function 1CH or 36H.
[1] The address returned in DS:BX points to a copy of the first sector of the actual FAT, with the media ID byte in the first byte.
[2.0+] The address returned in DS:BX points only to a copy of the media ID byte from the disk's FAT; the memory above that address cannot be assumed to contain the FAT or any other useful information. If direct access to the FAT is required, use Int 25H to read it into memory.
Example:
Determine whether the current disk drive is fixed or removable.
.
.
.
mov ah,1bh ; function number
int 21h ; transfer to MS-DOS
; check media ID byte
cmp byte ptr [bx],0f8h
je fixed ; jump if fixed disk
jmp floppy ; else assume floppy
.
.
.